Poplar River Turbidity Total Maximum Daily Load
Physical Channel Assessment
Task Order No. 2006-36
Prepared for:
U.S. Environmental Protection Agency Region 5
77 West Jackson Boulevard
Chicago, IL 60604-3507
Prepared by:
RTI International
3040 Cornwallis Road
Research Triangle Park, NC 27709-2194
USEPA Contract Number 68-C-02-110
February 25, 2008
Physical Channel Assessment of the Lower Poplar River
25 February 2008
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ACKNOWLEDGMENTS
Dr. Joe Magner (Minnesota Pollution Control Agency) provided essential direction and
guidance throughout the process including the field reconnaissance. Special thanks are due to
Ms. Julianne Socha, Project Officer-EPA, for her support with administrative aspects of this
grant funded project.
Several other experts contributed their time, efforts, and talent toward the preparation of this
report. The Project Team acknowledges the contributions of each of the following and thanks
them for their efforts:
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Mr. Tom Schaub (MPCA) for input during the preliminary field trip and coordination
efforts in the early stages.
Ms. Karen Evens (MPCA) for providing data and coordinating the communication
between the team and MPCA.
Mr. Clint Little (Minnesota Department of Natural Resources) for supplying essential
information, particularly the aerial photographs.
Ms. Debra Taylor (U.S. Environmental Agency – Duluth) for data sets on North Shore
summary.
RTI Project Team:
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Dr. Bill Cooter, RTI International – Raleigh, North Carolina
Mr. Doug Bach, SEH Inc. – Madison, Wisconsin
Mr. Brian Jacobson, URS Corporation – Raleigh, North Carolina
Mr. Troy Naperala, URS Corporation – Traverse City, Michigan
Mr. Dan Cazanacli, SEH Inc. – Minneapolis, Minnesota (Project Manager)
Dr. Sanjiv Sinha, Environmental Consulting & Technology (ECT) Inc. – Ann Arbor,
Michigan (Project Director)
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Table of Contents
1
2
3
4
5
6
7
8
EXECUTIVE SUMMARY............................................................................................................... 4
PROJECT INTRODUCTION: STUDY AREA AND FIELD RECONNAISSANCE ........... 6
2.1
General Characteristics ............................................................................................................ 6
2.2
Field Reconnaissance ............................................................................................................... 7
CHANNEL DESCRIPTION ............................................................................................................ 8
3.1
General Stream Characteristics ............................................................................................... 8
3.2
Flow Depth and Width ............................................................................................................. 9
3.3
Stream Valley and Channel Mobility................................................................................... 10
3.4
Erosion Processes within the Channel Banks and Valley Walls ...................................... 11
STREAM SEDIMENT AND ITS TRANSPORT ....................................................................... 12
4.1
In-Stream Sediment ................................................................................................................ 12
4.2
Suspended Sediment Size and Turbidity within Lower Poplar River ............................. 12
4.3
Possible Evidence of Increased Sediment Influx in Lower Poplar River ....................... 13
SUSPENDED SEDIMENT WITHIN LOWER POPLAR RIVER: SOURCES AND
ANNUAL ESTIMATES ................................................................................................................ 14
5.1
Sources and Processes Impacting the Amount of in Lower Poplar River ...................... 14
5.2
Estimating the Amount of Stream Related Suspended Sediment within Lower
Poplar River ............................................................................................................................ 18
CONCLUSIONS .............................................................................................................................. 20
REFERENCES ................................................................................................................................. 22
APPENDICES .................................................................................................................................. 23
List of Tables
Table 1.
Estimates of Annual Average Suspended Sediment ............................................... ..12
List of Appendices
Appendix A Maps of Study Area and Field Notes
Appendix B Field Photographs
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1 Executive Summary
A physical channel assessment was conducted to evaluate sources of localized soil erosion
along the Lower Poplar River. There were three primary purposes to this assessment,
namely: a) classifying the river to the extent possible, b) identifying the primary sources
and processes responsible for suspended sediment in the system, and c) quantifying the
input of suspended sediment from each of these sources and processes. The evaluation
presented here-in is expected to be preliminary for two reasons, namely: a) this report is
part of a larger initiative to establish a turbidity Total Maximum Daily Load (TMDL) for
the sub-watershed that looked at many parameters as well, and b) limited resources were
assigned for the field assessment.
The field reconnaissance included observations, cross section measurements and other data
collections. It was found the Lower Poplar River shares many characteristics with
mountain streams. Its longitudinal profile does not exhibit a gradual downstream decrease
in slope, typical of most streams. It is rather flat or slightly convex-up. A channel-reach
classification of mountain streams developed by Montgomery and Buffington (1997) was
used to characterize the stream. Four channel reach types (bedrock, cascade, plane-bed,
and step-pool) were identified. Most of the channel exhibits a morphology that is between
the step-pool type and plane-bed type. The bed surface is covered (armored) by gravel to
boulder size material. Larger, boulder-size particles, protruding above the base flow water
levels are ubiquitous throughout the channel. Due to the irregular bed surface the local
depth typically varies from several inches to approximately two feet during base flow
conditions. Pools have a maximum depth of three to four feet. Typically the width of the
channel ranges between 35 to 45 feet.
The channel lacks a well defined floodplain and, for the most part, is confined by the
topography of the valley. In places, the bottom of the valley is sufficiently wide and flat,
and the channel displays entrenched meanders and lateral mobility. Historical aerial
photographs indicate changes in channel position. One example is the meander loop at the
Megaslump site which has shifted over 200 feet. The banks are armored by irregular
ribbons of cobble to boulder material and there is little evidence of on-going channel bank
erosion. Dense vegetation dominated by trees and brushes, covers the surface above the
rock-armored sides with no well defined banks. The bank stability analysis discussed in the
QAPP, using Bank Stability and Toe Erosion Model, is not applicable and was not
performed.
The channel bed is dominated by poorly sorted large (coarse gravel to boulders) particles
and only a small proportion of finer sediment is found trapped in the spaces between larger
particles. In most places, only ten to twenty percent of larger particles are “buried” into the
finer sediment. Due to the stream’s high energy, there are no good geomorphic indicators
Physical Channel Assessment of the Lower Poplar River
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that would reflect an increased flux of fine sediment as described by Montgomery and
MacDonald (2002). Certain places such as the Megaslump were documented to have a
higher proportion of fine sediment in and near the streambed.
Six types of localized sediment sources were observed and are discussed in this document:
channel bed incision and stream lateral migration are found to not be a significant source
of fine sediment. However, when these processes occur in the vicinity of the valley slopes,
they can be a factor in the formation and expansion of landslides which in turn can
mobilize a large amount of fine, suspended sediment from the walls of the valley. The
erosion associated with a sudden channel migration such as meander cut-offs can mobilize
significant amount of material, including some fine, suspended sediment but their effect is
rather short lasting. Other potential sources of suspended sediment are within the river
valley and not directly related to stream activity. The erosion ravines formed at points of
concentrated runoff are an important source of suspended sediment. Finally, the
recreational development appears to have resulted in a multitude of surfaces that are
vulnerable to erosion. These include ski trails, ATV trails, hiking trails, utilities, road
surfaces, road embankments, and roadside ditches.
The report is organized as follows: Chapter 2 outlines the general characteristics of the
watershed. Description of the channel can be found in Chapter 3. A section on stream
sediment and its transport is outlined in Chapter 4. Chapter 5 discusses the sources of
sediment in the Lower Poplar River, and attempts to develop an estimate of their
contributions. Finally, Chapter 6 presents the conclusions of this investigation.
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2 Project Introduction:
Study Area and Field Reconnaissance
The goal of this physical channel assessment study was to better understand the evolution,
the present condition, and sources of turbidity within the Lower Poplar River. Analysis of
existing turbidity and other water quality parameters in the Poplar River revealed a strong
correlation between turbidity and total suspended solids (RTI 2007). The causes of
increased suspended sediment concentrations, whether episodic, temporary, or long term,
can be multiple and related to different erosion processes. The physical channel assessment
is aimed at estimating the stream related erosion areas and the relative contribution of
suspended sediment from each major source. Field observations and measurements were
focused on the stream itself, defined as the area of active flow (the streambed and the
stream banks) and the stream valley, a much larger (i.e., wider and deeper) feature that
evolved over geological time as a result of streambed incision and subsequent erosion
caused mainly by landslides.
2.1 General Characteristics
The Poplar River is one of the many streams on the North Shore of Lake Superior. Most of
these streams are relatively short and steep and have a steeper gradient in the lower
watershed than in the upper watershed (Fitzpatrick et al, 2007). This is due to the bedrock
topography and the adjacent terrain that was shaped by the last advance and retreat of icesheets. A ridge of Precambrian volcanic rocks, mostly basalts, parallels the Minnesota
shoreline of Lake Superior. The bedrock within the watershed of the Lower Poplar River
is relatively shallow and exposed in several places. The last glacial period, called
Wisconsinian (10,000 to 70,000 years ago), left behind thin drift of ground moraine
deposits.
The upper watershed of the Poplar River is located on an elevated plateau. The typical
elevation in the upper watershed is about 1300 feet and the average stream gradient is less
than 1 percent. The channel is relatively wide (100 feet or more) and characterized by wide
meanders. Dense vegetation consisting of willows, reeds, and other hydrophilic grasses
buffer the banks which show little signs of erosion. Impressive falls (rapids),
approximately 150 feet high, mark the transition from the upper watershed to the lower
watershed.
By contrast, the lower watershed, which is the topic of this assessment, is characterized by
a much steeper gradient and narrower width. For the purposes of this report, the “Lower
Poplar River” will describe the watershed area downstream of the Superior Hiking Trail
Bridge (Study Area Map).
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2.2 Field Reconnaissance
A field reconnaissance was performed between July 2 and July 6 of 2007. The scope of the
reconnaissance included field observations and measurements at fourteen locations (see
Appendix A). Photographs, soil or substrate characteristics, vegetation cover and other
observations were collected at these sites. The chosen locations were scattered and covered
all portions of the lower stream (upstream, middle, and downstream). In addition, width
and depth measurements as well as photographs were taken at all bridge crossings and
other access locations. At six of the locations, cross-sectional surveys and more detailed
observations were performed (Study Area Map, Figures 1, 2, and 3). The grain size
distribution was found to be bi-modal (i.e., a combination of very coarse material such as
boulders and cobble, and finer sediment trapped between larger particles). Therefore, a
precise determination of the median particle size (D50) was deemed to be of little use.
Some visual observation and limited measurements of the stream valley sides were also
made. All areas displaying visible signs of erosion were photo-documented. The large
landslide area referred to as the “Megaslump” (Figure 2) and, several other smaller
landslides and a major erosion ravine were identified in the upper portion of the lower
watershed. Study Area Map and Figures 1, 2, and 3 indicate the locations of the cross
sections, landslides and other important features that were documented as part of the field
reconnaissance. The field notes, cross-sections and a longitudinal profile are presented in
Appendix A. Photographs are appended in Appendix B.
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3 Channel Description
3.1 General Stream Characteristics
The Lower Poplar River has more in common with mountain streams than with the typical
lowland streams of the Midwest. Like many of the mountain streams, the Lower Poplar
River does not fall into a general category of braided or meandering streams. Braided
streams are characterized by channel bars and flow splits, a result of intense sediment
deposition. Typical meandering streams evolve as a result of the flow interaction between
the channel and the adjacent floodplain; an interaction defined by continuous channel
lateral shifting and overbank deposition. Neither process is typical of the Lower Poplar
River. Furthermore, the longitudinal profile does not exhibit a typical concave-up shape
commonly referred to as a graded profile (i.e., the longitudinal bed slope is steeper
upstream and gradually milder downstream). A sharp change in bed elevation is noticeable
near the mouth where a succession of falls is present (upstream and downstream of
Highway 61). Upstream from these falls, the average longitudinal slope is approximately
0.03 (3 percent) and the general shape of the profile is rather flat or slightly convex-up (see
Appendix A). Such longitudinal shapes are common in cases of relatively young rivers
developed in glacial valley. Ellen Wohl, in a comprehensive review of mountain rivers
(2000), noted that “Mountain river channels are less likely than low-gradient alluvial
channels to approximate the graded longitudinal profile and progressive downstream
changes in channel morphology that are commonly regarded as the stable conditions
toward which rivers trend…. Because of the relative lack of consistent downstream trends,
mountain rivers are most appropriately classified at reach scale”. Essentially this means
that the characteristics of these rivers may change somewhat irregularly from one portion
to another. Following this idea, it seems more fitting to use the process based channelreach classification of mountain streams of Montgomery and Buffington (1997). The
authors identify seven types of channel reaches that are commonly encountered in
mountain drainage basins: bedrock, cascade, step-pool, plane-bed, pool-riffle, dune-ripple,
and colluvial. These channel reach types can be found in progression in many of the
mountain streams. In the case of the Lower Poplar River, four of these channel reach types,
namely bedrock, cascade, plane-bed, and step-pool were identified. An explanation of each
of these four channel reaches follows.
Bedrock reach is a special channel type in a sense that it displays little to no alluvial
material. The channel is incised into bedrock and confined by steep valley walls. The
channel geometry and the position of the bed are controlled by the rate of incision. Two
significant bedrock outcrops were identified along the Lower Poplar River. One is a
succession of falls approximately 500 feet to 1500 feet upstream from the mouth extending
south and north of Highway 61. The other one is a reach located approximately a mile
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from the mouth, on the north edge of the golf course (Study Area Map - Appendix A;
Photo 11 – Appendix B).
The cascade type is only present in the upstream part where the steep channel bed is
confined within a relatively narrow valley. In cascade type reaches, the flow tumbles over
large boulders that dominate the channel bed. The size of the boulders exceeds the depth of
the flow several times.
The step-pool and plane-bed types are by far the most common channel-reach types along
the Lower Poplar River. Overall, the channel exhibits a morphology that is between the
step-pool type and plane-bed type. The step-pool type (Photo 10 – Appendix B) is defined
as a rhythmic sequence of drops followed by a scour pool. Only the upstream part shows
regularity in terms of spacing with steps spaced a fraction of the channel width apart. The
remainder of the river shows little regularity in terms of steps distribution. Most of the
pools are relatively shallow (less than 3 feet deep). The plane-bed term is also applied to
streams with sandy bottoms which could possibly lead to confusion. In their classification
of mountain streams, Montgomery and Buffington (1997) use this term to describe streams
with much coarser (gravel to boulder) material. This definition is also applicable to the
Lower Poplar River where the bed surface is covered (armored) by gravel to boulder size
material (Photo 9 – Appendix B). Larger, boulder-size particles, protruding above the
normal water levels are ubiquitous throughout the channel.
The coarse material is colluvial in origin, meaning that it is detached from the valley walls
and transported downhill by gravity (i.e., in conjunction with sloughs or landslides).
Examination of the valley walls in places that lack vegetation cover (i.e., landslides) shows
a poorly sorted mixture of boulders and cobbles encased in a matrix of finer sandy and
silty material (Photo 13 – Appendix B). This mix is typical of glacial deposits. The size
range and composition of the cobbles and boulders observed along the valley walls
matches those of the coarse material observed within the stream.
3.2 Flow Depth and Width
The ubiquitous nature of boulder particles, many of which protrude above the water
surface, translates into irregular shape cross-sections making the flow depth measurements
difficult. On average, the typical base flow depth is slightly over one foot (Appendix A –
Cross Sections). Within a given cross-section, the local depth typically varies from several
inches to approximately two feet. Narrow portions of the channel and infrequent pools may
show a maximum depth of three to four feet. The only place where a deeper pool is present
is near the mouth, downstream of Highway 61.
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Simple flow computations indicate that, on average, during high flow periods, which
typically occur late in the spring and are caused by snow melt, the water levels could be 1
to 3 feet higher than the normal flow conditions observed during the field reconnaissance.
During base flow conditions, the flow width of the Lower Poplar River varies from 30 to
60 feet with the majority of the measurements falling within 35 to 45 feet range. The
coarsest streambed material ranges from large blocks (5 to 10 feet) at the upstream end to
boulders and cobble (0.5 to 2 feet) in the downstream part (Appendix B – Photos).
3.3 Stream Valley and Channel Mobility
The Lower Poplar River flows through a valley that is approximately 120 to 250 feet deep
and 500 to 1000 feet wide. The average side slopes of the valley (where many of the ski
trails are located) vary between 10 and 25 degrees (18% to 50%). The bottom part of the
valley is, however, less steep and varies in width from 100 to 400 feet (Appendix A –
Study Area Map).
The channel lacks a well defined floodplain and, for the most part, is confined by the
topography of the valley. In some places, one side of the channel is flanked directly by the
valley slopes. In other places, however, the bottom of the valley is sufficiently wide and
flat and it could be looked upon as a narrow floodplain. The channel displays some lateral
mobility and several entrenched meanders have developed as a result. Entrenched
meanders develop in places where floodplains are relatively narrow and their surface slope
towards the channel (i.e., as opposed to wide and flat floodplains that allow for highly
sinuous meanders to form).
Examination of historical aerial photographs indicates some changes in channel position,
most notably in the upstream part. Aerial photographs from 2003 compared against aerial
photographs from 1934 reveal that a major meander loop, where the Megaslump is located,
migrated south-southeast by approximately 220 feet (Appendix A – Figure 2). Other major
changes are a smaller meander migration further upstream in the ski hills area, and a
couple of meander cut-offs, located within the downstream portion, approximately half a
mile from the mouth (Appendix A – Figure 1). The field reconnaissance documented
several remnants of abandoned streambeds. The segments that have been abandoned were
filled with relatively well sorted cobble and their surface is above the surface of the present
channel bed. This suggests that the channel migration took place in conjunction with
channel incision. Unlike the meanders of low-gradient alluvial streams with wide
floodplains, the Lower Poplar River’s meanders are entrenched and well confined within
the valley.
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3.4 Erosion Processes within the Channel Banks and Valley
Walls
The sides of the active flow area are armored by irregular ribbons of coarse size (cobble to
boulder) of densely packed rock material. As the erosion of the valley progressed, the bulk
of the finer material has been transported downstream, while the boulder and cobble size
particles have been retained within the channel stabilizing its sides and bottom. Thus, the
erosion and selective transport processes resulted in a self-armoring mechanism. There is
very little evidence in terms of on-going channel bank erosion (Appendix B – Photos 8, 9,
10, 11). Erosion is visible, however, in places where the bank and the valley wall form a
continuous slope surface, most notably in the Megaslump area where the channel is
adjacent to the sloped surface of this major landslide (Appendix B – Photo 14). This and
other landslides are a different form of erosion from what it typically referred to as stream
bank erosion. The distinction between stream bank erosion and valley wall erosion is not
simply semantic. Stream bank erosion typically implies a local and limited slope failure
often aided by the collapse of trees. It is a common occurrence along alluvial channels
which tend to have well defined banks with a top surface that is periodically flooded. Thus,
the vertical extension of the stream bank erosion surface is limited. By contrast, the
landslides documented along the Lower Poplar River extend vertically for tens of feet, well
above highest flooding levels. The likely mechanism that triggers the formation of these
landslides is the erosion of the slope toe due to channel incision and lateral migration. The
same processes are also likely to cause an expansion of the landslide area.
On average, the rock-armored sides of the channel extend 1 to 2 feet vertically and 5 to 20
feet laterally. Dense vegetation dominated by trees and brushes, covers the surface above
the rock-armored sides. These vegetated surfaces are less steep but for the most part, there
is no clearly identifiable top of the bank. In many places the side slopes of the channel and
the adjacent slopes form a continuum with little or no slope inflection. Therefore, the
geometric dimensions of the channel banks cannot be easily defined. The bankfull depth, a
popular geometric dimension employed in the characterization of self-formed alluvial
channels with well defined floodplains, is neither identifiable nor relevant in the case of the
Lower Poplar River since, for the most part, the channel does not exhibit well defined
banks. For these reasons, the type-bank stability analysis discussed in the QAPP, using
Bank Stability and Toe Erosion Model, is not applicable and therefore was not performed.
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4 Stream Sediment and its Transport
4.1 In-Stream Sediment
As mentioned above, the channel bed is dominated by poorly sorted large (coarse gravel to
boulders) particles. The largest of these particles are irregularly spread across the channel
and, during base-flow conditions, protrude above the surface of the water. A small
proportion of finer sediment, mostly coarse-sand to fine-gravel in size, is found trapped in
the spaces between larger particles. Another way to define this spatial relationship is to
look at the larger particles as being superficially buried into the finer sediment. The degree
to which the larger particles are “buried” into the finer sediment is referred to as
embeddedness. Quantitative embeddedness measurements are labor intensive, time
consuming and logistically difficult to execute in a rapid flowing stream. In Lower Poplar
River, based on visual observation, in most places, only 10 to 20 percent of the volume of
the cobbles and boulders is buried into the finer sand to gravel sediment bed. This ratio
applies both to the streambed and the stream banks.
The coarse cobble-size material can only be entrained and rolled downstream during high
flow conditions. The larger, boulder-size particles are not likely to be entrained at all,
except during extremely large flows. The stream has more than adequate energy
(competence) to transport the sand-sized and finer particles during normal flow conditions.
The transport of fine material is essentially supply-limited. In other words, the stream can
transport all of the sand-sized, and any finer material reaching the channel. The transport
of fine material is essentially supply-limited. In other words, the stream can transport all of
the sand-sized, and any finer material reaching the channel. The presence of finer material
within the bed and along banks is due to trapping between much larger particles. Placed
between much larger particles, the smaller sand-sized particles are essentially sheltered
from the drag force imposed by the flow, a process referred to as the “hiding” effect.
During increased flow conditions, the increased drag force and small scale turbulence may
entrain a small fraction of the sand-sized sediment that is trapped between the larger
particles. More sand is likely to be entrained during the spring high flows when some of
the large cobble-size particles that shelter the fine sediment are mobilized as well. Thus,
the channel itself is not a source or a repository of fine sediment. The fine sediment is
limited in supply and periodically flushed out.
4.2 Suspended Sediment Size and Turbidity within Lower
Poplar River
One of the main questions to be addressed as part of the overall TMDL process is whether
and how the channel processes may impact the level of turbidity. The exact size
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distribution of the suspended sediment that results in elevated levels of turbidity is not
known. However, it is reasonable to assume that the bulk of the suspended sediment
responsible for the turbidity levels falls within silt and clay size range. This is similar to a
limited set of suspended sediment grain size data for the Baptism River available from the
USGS that indicates , on average, silt size and clay size sediment make up 85% of the total
suspended load. The Baptism River is located southwest from the Poplar River and has
similar characteristics. Based on visual observations within the lower Poplar River
watershed the bulk of the fine sediment found along the Lower Poplar River channel bed
and banks is coarser, falling in the range of medium size sand to fine gravel with a very
small proportion of silt and clay particles. Substrate particle size data from other North
Shore streams with gravel to boulder beds indicate the proportion of clay, silt, and fine
sand is in the substrate is less than 2 percent (US EPA). All this information indicates that
the channel itself is not a significant source of fine (i.e., mostly clay and silt size)
suspended sediment that would in turn result in elevated turbidity levels.
4.3 Possible Evidence of Increased Sediment Influx in Lower
Poplar River
Montgomery and MacDonald (2002) provide a summary of the stream characteristics that
are indicative of a “chronic increase” in the supply of fine sediment (i.e., particles smaller
than 2 mm). These characteristics include channel dimensions, bed material, pool
characteristics, reach morphology, and sediment transport (suspended load and bed load).
Documenting changes in these characteristics implies measurements performed at different
moments in time. Unfortunately, no previous detailed investigation is available in case of
the Lower Poplar River. According to Montgomery and MacDonald, the cascade and steppool reach types show only limited changes in these characteristics when the supply of fine
sediment increases. The plane-bed reach type would respond to a chronic increase in fine
sediment by displaying an increase in fine sediment in the bed material and increased
embeddedness. Because of the high energy flow, an increase in fine sediment proportion
along the entire channel is not to be expected. Unless trapped between larger particles all
fine sediment is eventually carried downstream. Certain places, however, were
documented to have a higher proportion of fine sediments and a higher embeddedness.
These places include the Megaslump (Appendix B – Photo 15), another landslide located
further upstream, and a large size erosion ravine discussed below (Appendix B – Photo
30).
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5 Suspended Sediment within Lower Poplar River:
Sources and Annual Estimates
5.1 Sources and Processes Impacting the Amount of
Suspended Sediment in Lower Poplar River
In general, the potential localized sources of suspended sediment in the Lower Poplar
River watershed include:

Channel bed incision

Sudden channel migration (e.g., meander cut-off, channel avulsion, etc)

Streambank erosion

Landslides (slumps) near active channel

Incision along valley slopes (erosion gullies and ravines)

Localized erosion within the river valley related to land-use alteration
The more uniform, watershed-wide erosion due to detachment of soil particles is analyzed
separately and is not included in this report. The first three sediment sources listed above
are directly related to the stream and stream processes. The fourth source refers to
sediment detached from the valley walls but, the stream processes, streambed incision and
lateral migration specifically, may play a role in triggering and enlarging these landslides.
The last two possible sources are located within the valley sides and are largely
independent of the processes within the main stream.
The assessment of sediment sources is further complicated when potential anthropogenic
factors may exist. The current human factor is not likely to significantly impact the stream
erosion rates in the area of active flow (i.e. streambed incision or channel lateral
migration). However, land use practices, such as recreational development, may have
contributed to: a) expansion of landslide areas, b) formation of erosion and ravines gullies
on the valley sides through creation of concentrated flow routes, and c) larger than normal
sediment erosion rates within the watershed due to changes in land-use resulting from
removal or degradation of the vegetation cover.
Individual sources and processes of suspended sediment are discussed in further detail
below:
Channel Bed Incision
Channel incision is an ongoing geological process which characterizes all high-gradient
North Shore streams flowing into Lake Superior. These streams continue to cut down in
the glacial till material, slowly adjusting the shape and slope of the longitudinal profile.
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This process, however, takes place at a relatively slow rate and, on an annual basis, will not
result in the mobilization of significant amounts of suspended sediment from the
streambed. While the channel bed itself is not a significant source of suspended sediment,
channel incision may play a role in the occurrence of landslides observed in the vicinity of
the channel. Channel bed incision may occur simultaneously with the gradual streambank
migration discussed below.
Sudden Channel Migration (e.g. Meander Cut-Off)
Aerial photographs taken in 1934, 1991, and 2003 suggests that rapid lateral migration has
taken place at certain locations. The meander cut-offs and channel diversions have likely
entrained a significant amount of sediment including a suspended load fraction. However,
these kinds of sudden channel migrations are one-time events associated with abnormally
high flow rates. Most of the suspended load that would have been generated this way was
dispersed throughout the stream and flushed out of the system. Empirical evidence and
laboratory study indicate that pulses of fine sediment in streams are dispersed rather fast
(Cui et al., 2003).
Streambank Erosion (Gradual Channel Migration)
Stream bank erosion implies gradual channel migration, as opposed to major sudden
changes, such as channel avulsions or meander cut-offs, which are one time processes
associated with extreme flow events. Generally speaking, stream bank erosion in alluvial
streams could greatly increase the amount of suspended load due to the local degradation
and collapse of the banks. However, in the case of the Lower Poplar River the banks are
armored with large size particles and there is little evidence of active, on-going bank
erosion.
Another aspect that deserves attention is the erosion associated with large falling trees.
When a tree tilts toward the channel and ultimately falls down, the root system dislodges
sediment from the bank. A qualitative visual assessment suggests that the large falling
trees as well as the large woody debris are rather infrequent. On average, one major falling
tree was observed every several hundred feet. Except for historic logging
Landslides Near Active Channel
In places where the active channel is near the valley wall, landslides are likely to form
and/or become larger as the channel shifts laterally. Such places, most notably the meander
bend at the Megaslump, could be considered the equivalent of the gradual streambank
erosion except that it is the slope of the valley that is being eroded and not the streambank
(Appendix A – Figure 2).
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Several landslides have been documented along the channel. The Megaslump, is much
larger in size and its total surface area may exceed the surface area of all the other
landslides combined (Appendix A – Study Area Map). The Megaslump is located on the
eastern bank approximately 1½ miles upstream from the mouth. At this location, the
channel forms a relatively tight meander that could be tracked on the aerial photography
from 1934 which also shows what appears to be a landslide of considerably smaller size.
Since then, the meander appears to have shifted approximately 220 feet in a southsoutheast direction, impinging upon the relatively steep side of the valley. Presently, the
stream flows adjacent to the slope failure area which is approximately 50 feet high. Water
discharge from the nearby water treatment ponds may have contributed to the expansion of
the exposed failure surface. Erosion of the Megaslump will be minimal if the discharge
operates as designed; however, if the flexible discharge pipe were to become detached
during discharge event significant erosion would occur. Several gullies dissect the face of
the landslide and the unvegetated soil surface appears to be highly erodible (Appendix B –
Photo 13). Given its size and proximity to the channel, the Megaslump area is a likely
major source of fine sediment. The translation of the eroded surface mentioned above
suggests that the sediment delivery mechanism was a progressive slope failure, the
collapsed material being washed into the stream. Stream observations in the vicinity of the
Megaslump reveal that the embeddedness is above the average (Appendix B – Photo 15).
The larger cobble size particles are buried into finer sand-sized sediment in a proportion 25
to 40 percent suggesting a higher influx of sediment.
Two other landslides, both smaller in size than the Megaslump, were documented in the
east side of the valley. In the order of their relative size, these landslides are located 1) a
short distance downstream of the Megaslump on the east side (Appendix B – Photo 12)
and 2) in the upstream ski hill area (approximately 2 miles upstream from the mouth) also
on the east bank along a major meander bend (which based on aerial photographs migrated
approximately 80 feet towards southeast between 1934 and 2003). In the vicinity of each
of these landslides and a short distance downstream, the proportion of finer sediment
trapped in the streambed (i.e., embeddedness) is higher than the typical average suggesting
on-going erosion at all of these places.
Other landslides, even smaller in size, are located in an upstream forested area where there
has been little to no change in the land use. A series of three near-channel landslides have
been documented within a short distance downstream of the Poplar Rapids, all on the west
side of the valley (Appendix A – Figure 1 and Appendix B – Photos 27 and 28). This area
is densely forested. Field observations and aerial photographs indicate no logging or other
type of land alteration in recent years.
This suggests that landslides could occur regardless of the changes in land use. To verify
this assertion, a short trip to the Onion River, documented a landslide of considerable
Physical Channel Assessment of the Lower Poplar River
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dimensions (Appendix B – Photo 34). The Onion River is a smaller stream flowing into
Lake Superior with a watershed that is densely forested and experienced little to no land
use alteration.
Incision along Valley Slopes – Gullies and Ravines
Gullies and ravines are common erosion features in places of concentrated runoff along
steep slopes. Such features are a common natural occurrence and part of the drainage basin
denudation process. However, the gullies that are naturally occurring evolve relatively
slowly into ephemeral tributaries to the main stream. By contrast, the erosion gullies that
formed as the result of increased runoff from developed areas are fast evolving and can
mobilize large amounts of sediment.
There are several places of concentrated runoff from storm events within the Lower Poplar
River valley that occur as a result of the recreational-based development (ski trails, ATV
trails, access roads, ski lifts, resorts, facility buildings, etc). In some places, significant
efforts have been made in recent years to limit, eliminate, or mitigate the gully erosion and
to convey the runoff flow to the stream in a controlled, non-erosive way. Most notably, the
runoff from the eastern tributary valley near Eagle Mountain where many of the ski trails
and local roads are located is routed across a series of swales. These swales were
landscaped across the slope with the apparent intent to break the slope surface into smaller
segments and control the erosive power of the runoff (Appendix B – Photo 23). Small size
drains have been installed along the swales and efforts to vegetate and stabilize the sloped
surfaces between swales have been made.
A place of concentrated runoff that resulted into a large erosion ravine was identified in the
very upstream part of the river. A 320-foot long ravine spans through a forested area from
the north end of the main road (Ski Hill Road) to the edge of the stream (Appendix A –
Figure 1). This ravine is approximately 10 to 20 feet deep. The average longitudinal slope
is approximately 21 degrees (40%). To a certain extent, the bottom of the ravine appears to
be have been reinforced with debris and boulder rock material. The side slopes, however,
are unvegetated and very steep (over 45 degrees) with potential for future soil erosion
(Appendix B – Photo 29). Given its size and proximity to the river, this erosion ravine is
likely to contribute significant amount of fine sediment to the stream. The streambank at
the bottom of this ravine shows a higher than average proportion of finer sediment (i.e.,
higher embeddedness, approximately 25 to 30 percent; Appendix B – Photo 30).
Another erosion ravine, smaller in size (approximately 180 feet long and 10 feet deep) is
located in the main ski area on the east side of the valley. It extends from a ski lift post to
the most upstream bridge before the rapids (Appendix A – Figure 1; Appendix B – Photo
22).
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Localized Erosion within River Valley
To some extent, all sloping surfaces along the Lower Poplar River valley are subjected to
erosion processes that could result in fine sediment being delivered into the main stream.
This applies even to some well-vegetated and forested areas that were not affected at all by
human activities. However, those surfaces where land-use conversion from forest took
place became more vulnerable to erosive processes and are likely to contribute a
disproportionate amount of sediment to the stream. Based on field observation, several
types of surfaces appear to be particularly vulnerable to erosion and may represent a major
localized source of suspended sediment. These types of surfaces include ski trails, ATV
trails, hiking trails, utilities road surfaces, road embankments, and roadside ditches
(Appendix B – Photos 16, 17, 32). Additionally, several landslides, at appreciable
distances above and not directly related to the main stream, have been documented
(Appendix B – Photo 24).
5.2 Estimating the Amount of Stream Related Suspended
Sediment within Lower Poplar River
An approximate estimate of the annual average suspended sediment contributions from
sources outlined in Section 5.1 is presented below. These geometric volume estimates are
based on aerial photographs, historical (1934) and current (2003), and on topographic data
(contours) coupled with field observation. For each area that appears to be eroded, or
“washed away” by river migration, a minimum and maximum estimate of the length and
width was determined through direct measurements from the map. The range of estimates
account for and accommodate, the resolution and accuracy of the aerial photographs. The
height is based on field observations of erosion surfaces.
There are two major challenges to accurately estimate eroded volumes. First, it is difficult
to estimate the exact timeframe over which the erosion took place. Second, but perhaps
more important, the percentage of eroded material from each source that actually became
suspended sediment is unknown. It is assumed that the sediment fraction related to the
turbidity primarily consists of clay, silt, and fine sand.
Based on field observation at the Megaslump and other landslides areas, the red clayey
soils dominating the valley walls, have a considerable fraction of fine material that, once it
reaches the stream, would become suspended sediment (i.e., mostly clay and silt). Based
on visual observations, about one third of soil volume in the valley wall seems to fall in the
clay and silt size range. A 25 to 40 percent range was used for the estimate presented in the
table below.
On the other hand, the streambed and stream banks contain very little fine sediment. The 5
to 10 percent range used in the table below is based on the assumption that the material in
Physical Channel Assessment of the Lower Poplar River
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the areas adjacent to the active flow, where the stream could potentially migrate, (i.e., the
bottom of the valley that could be regarded as a narrow floodplain) consists primarily of a
matrix of boulder and cobble. Thus, when the channel migrates laterally, less than half of
the scoured material falls within clay to fine gravel range, of which a smaller fraction
potentially resulted in suspended sediment.
Based on these estimates, the following table is presented as a preliminary assessment of
suspended sediment amount in the Lower Poplar River.
Table 1. Estimates of Annual Average Suspended Sediment (Only from Stream Sources)
Eroded Volume
Estimates
(CY)
Min
Max
Channel Incision and
Gradual Lateral
Migration*
16,593 41,111
Megaslump
57,037 85,556
Other Landslides
22,500 33,333
* Meander cutoffs not included.
Typical
Bulk
Density
(pcf)
110
110
110
Proportion of
Eroded Soil that
becomes
Suspended
Sediment %
Min
Max
5%
25%
25%
10%
40%
40%
Time
Interval
(years)
1934 - 2003
69
69
69
Average Annual
Suspended
Sediment Flux
(Tons/year)
Min
Max
18
307
121
The estimates, while approximate, suggest that channel itself represents only a minor
source of suspended sediment while the landslides could generate an amount of suspended
sediment that is an order of magnitude higher.
Rough estimates of suspended sediments from the major erosion ravines indicate a total
quantity between 1500 and 2800 tons. However, the age of these erosions ravines is largely
unknown. It could vary between several years to forty years (i.e., since the beginning
expansion of skiing areas) resulting in annual suspended sediment rates that could range
from 40 to 500 tons. It would be fair to conclude that the largest erosion ravine
documented upstream of the ski hill is relatively young and represent a significant source
of sediment.
88
737
287
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6 Conclusions
The physical channel assessment was conducted during a single visit to the watershed
lasting 4 days. Estimates of sediment loading to the Poplar River were based on field
observations and photographs taken during the site visit and compared with historical
photographs and maps. This limited field investigation limits the certainty of the estimates
to the range of values reported.
Stream Type
The Lower Poplar River is a steep and high-energy, mountain stream. The active channel
lacks a floodplain and is largely confined within a relatively steep and narrow valley. The
bottom width of the valley varies, allowing for some local channel mobility in the form of
gradual lateral migration coupled with channel bed incision. There is also evidence of
sudden migration in form of meander cut-offs. The channel bed and banks are dominated
by coarse, cobble and boulder size, material. The fine sediment transport capacity exceeds
the supply from the valley walls. A limited amount of finer bed material, predominantly
coarse sand to fine gravel is trapped between the larger particles.
Stream Sediment
The channel bed and banks are dominated by coarse, cobble and boulder size, material.
The fine sediment transport capacity exceeds the supply from the valley walls. A limited
amount of finer bed material, predominantly coarse sand to fine gravel is trapped between
the larger particles. Given the strongly bi-modal sediment distribution, a D50 analysis was
not carried out.
Primary Sources of Turbidity in Lower Poplar River
Six sources of turbidity in the stream are identified and discussed in Section 5.1. These are:

Channel bed incision

Sudden channel migration

Stream-bank erosion

Landslides near active channel

Incision along valley slopes

Localized erosion within river valley
Of these sources, our preliminary assessment indicates primary long term sources of
turbidity in the system are landslides, sudden channel migration, and incision along valley
slopes.
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Evidence of Landslides as Natural Processes
Given the interest in understanding the anthropogenic effect on the system, it is important
to outline that within the Lower Poplar System, there is good evidence that landslides
occur as a natural process, irrespective of land alteration. However, in the case of the
Megaslump, land alteration and concentrated runoff in recent years may have increased the
amount of erosion and sediment that was dislodged from the valley wall and delivered into
the stream. Collectively, the landslides are a significant source of sediment including an
important fraction of suspended sediment. A multitude of other places susceptible to
erosion within the river valley can be attributed to land-use changes, development of the
multi-use trail system, and general grading operations. Collectively, these places represent
a significant source of suspended sediment and include ski trails, ATV trails, utility/access
roads and their embankments, roadside ditches and some of the landslides located further
away from the stream.
Quantity of Suspended Sediment
Our preliminary assessment presented in Table 1 indicates that the largest source of
suspended sediment in Lower Poplar near-stream system is from the Megaslump and the
other landslides. The Megaslump feature appears to have contributed nearly 60-70% of
the suspended sediment in the system. Channel incision and lateral migrations appear to
provide the rest of the suspended sediment in the system. Finally, as indicated in the
Executive Summary of this document, it is important to underscore that the
geomorphology assessment is only one piece of a puzzle of many pieces. A more accurate
estimate of suspended sediment loading is expected to be an outcome of the modeling
component of this TMDL project, and observations presented here-in are meant to
establish markers for that study.
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7 References
Cui Y., Parker, G., Pizzuto, J, and Lisle, T.E., 2003. Sediment pulses in mountain rivers; 3.
Comparison between experiments and numerical predictions. Water Resources Research, v. 39 No.
9, p. 4-1:4-11.
Fitzpatrick, F.A., Peppler, M.C., DePhilip, M.M., and Lee, K.E. 2006. Geomorphic Characteristics
and Classification of Duluth-Area Stream, Minnesota. USGS Scientific Investigation Report 54 pp.
Montgomery, D.R. and Buffington, J.M., 1997. Channel-reach morphology in mountain drainage
basins. GSA Bulletin, v.109, p. 596-611.
Montgomery, D.R. and MacDonald L.H., 2002. Diagnostic Approach to Stream Channel
Assessment and Monitoring. Journal of American Water Resources Association, v. 38, p. 1-15.
RTI, 2007. Poplar River Turbidity Total Maximum Daily Load Evaluation of Existing Data.
Prepared for US EPA Region 5, 77 West Jackson, Chicago, IL 60604. Prepared under contract 68C02-110. August 16, 2007.
Wohl, E.E., 2000. Mountain Rivers. American Geophysical Union, 320 pp.
Physical Channel Assessment of the Lower Poplar River
8 Appendices
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Physical Channel Assessment of the Lower Poplar River
Appendix A
Maps of Study Area, Field Notes,
Longitudinal Profile and Cross Sections
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Physical Channel Assessment of the Lower Poplar River
Appendix B
Field Photographs
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