River Evolution Component
Rose Wallick
Last Modified: June 7, 2004
Key Question(s) Addressed
 What attributes of channel change are most important in Evoland?
 How should Evoland account for channel change due to natural & anthropogenic factors?
Proposed General Approach
This paper presents a highly simplified mechanism for determining the migration potential of the
active channel. We assume that migration (& avulsion) leads to increased channel complexity,
thus metrics describing migration potential also describe the likelihood of channel
‘complexification’. Channel movement is completely theoretical; for each time period, attributes
pertaining to erosion and migration are updated for 1 km slices of the active channel, but there is
no net change in channel position.
Background and Justification
We assume that ‘natural’ evolution of the Willamette would proceed as follows:
1. Lateral migration will eventually increase bend sinuosity
a. Migration is most rapid along areas flanked by Holocene alluvium; migration is
probably more rapid through unvegetated lands.
2. As bend-sinuosity increases, there is a higher likelihood that the bend may be abandoned
through avulsion
a. Avulsions are facilitated by peak flows, large wood accumulations and high
sediment load (Wallick, 2004, O’Connor, 2003).
3. Erosion will lead to deposition of point bars, mid-channel bars or island growth
a. These ‘new’ surfaces may be colonized by vegetation & will increase habitat
diversity (Dykaar and Wiginton, 2000).
4. Migration & avulsion will create an array of different geomorphic surfaces. Many
recently abandoned or created surfaces will be topographically lower than older surfaces
& will be inundated at high flows—thus allowing for creation & maintenance of offchannel habitat.
Along the late 20th century Willamette River:
1. Migration & avulsions are limited by revetments (Wallick, 2004; Gutowsky, 2000)
2. Migration is most rapid in un-revetted areas flanked by Holocene alluvium
3. Difficult to quantify erodibility of different vegetation types; but work by others suggests
vegetation may stabilize banks (e.g., Thorne, 1990; Simon, 2002).
Implementation Details
1. Create river_slices.shp file for study area
a. File contains 1995 active channel polygons for the Willamette & major
tributary (e.g., McKenzie) intersected with slices coverage for the study area.
b. Columns are added to the data table so that each river slice is attributed with
the following ‘static’ information:
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i. Biocomplexity Reach (refers to study area)
ii. River_name (designates whether slice is on Willamette or tributary)
iii. Historical ranking of channel dynamics (will be explained further-see
(Section 2 below)
iv. Geological control (Null entry-indicates if there is large-scale
geological control, such as resistant bedrock, that would diminish
migration).
c. Dynamic columns to be updated at each time-step include:
i. LULC (some average LULC_A value to indicate how landcover might
influence bank erodibility)
ii. Revetment (indicates whether or not there is revetment along the
channel in each slice)
iii. Large wood – a variable describing the local availability of large
wood. This will be a function of local erosion during the previous
timestep & upstream inputs. Still working on the algorithm to
calculate available large wood
d. From (b) and (c) the model determines whether channel movement is likely &
the extent of migration (see Section 3 below).
2. Determining Historical Migration Ranking-2 Possible Approachesa. Option 1: easy, qualitative assessment
i. Overlay 1850, 1895, 1932, 1972 & 1995 channels for mainstem
Willamette
ii. Assign relative values of historically stable, moderate historical
erosion & highly dynamic to each slice.
b. Option 2: more quantitative assessment (not sure if it will work with small
sample sizes)
i. Calculate average annual distance of bank erosion per slice per time
interval for Non-revetted areas (if revetment present don’t include
1972-1995 time period)
ii. Calculate mean-annual erosion distance for the slice from values
computed in (i)
iii. From the population of erosion distances in (ii) calculate preliminary
statistics to determine if it is feasible to separate the slices population
into 3 general categories: historically stable, moderate historical
erosion & highly dynamic (numbered 1-3).
c. Problem with both approaches is that they are not applicable to tributary
reaches & that historic erosion may not be best predictor of future channel
movement. Also, measuring erosion distances for avulsion vs. migration is
problematic (e.g., large eroded distances for avulsions may skew statistics).
3. Determining the likelihood & relative amount of migration
a. If revetment is present in a particular river slice, we assume migration in that
slice is likely negligible. This isn’t entirely correct; In actuality, the channel
sometimes migrates away from revetment-but without calculating length of
revetment relative to channel length in each slice, it is difficult to tell how
‘stabilized’ a particular slice may be. Therefore, it seems simplest to state
that migration will be negligible in any slice where revetment is present.
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b. Local geology can essentially stabilize a reach and cause local migration to be
minimal. This is where we would use the geological control attribute-so far,
I’ve only came across 1 area like this.
c. If no revetment or geological control, migration is possible & we calculate
the relative amount of migration by combining the historic ranking, LULC &
wood scores
i. Use LULC to assign relative erodibility ranking on a scale of 1-3.
Low erodibility LULC’s (e.g., forest) receive a rank of 1 and high
erodibility LULC’s (bare ground, agriculture, wetlands, water) receive
a score of 3. Moderately erodible LULC’s may be natural
vegetation…I’m open to suggestions on this-should we only have 2
categories instead of 3?
ii. Wood…still working on this algorithm –basic idea is that slices with
lots of large wood will be more likely to display dynamic behavior
than similar slices lacking wood. (e.g., lots of wood =score of 3; little
to no wood = 1).
iii. Overall migration score is then obtained by averaging the historical
ranking, LULC erodibility ranking.
Data Needs
 River slices-currently using 1995 active channel for Willamette & major tribs
 Historic migration score-to be computed by Rose for each study area
 LULC A –need some average LULC score for each river slice
 Geologic map – for geologic score; currently using O’Connor (2001)
Issues and Questions for the Workshop
Questions re: Simple model of river evolution
 Is this simple model an accurate representation of channel change?
 Can we assume that if revetment is present along a 1 km river slice, erosion will be
negligible? Alternatively, should we calculate the length of revetment per length of
channel & determine the extent to which revetment may stabilize a particular river slice?
 How should we deal with revetment removal? If revetment borders several parcels, can 1
actor remove revetment from his parcel?
 How should we score the ‘erodibility’ of LULC classes? How many categories of LULC
erodibility should we have?
 How should we deal with large wood? Ideally, if forest gets eroded, we should update
local & downstream cells with a ‘wood flag’. This could be also be used in ecosystem
health model.
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Questions re: Fundamental aspects of riparian areas in Evoland
How do we define ‘riparian area’ in Evoland?
What is the Independent Decision Unit (IDU) for riparian areas?
How do we link channel change/disturbance with other aspects of Evoland (e.g,
Ecosystem Health model, actor decisions etc)
References
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Dykaar, B.B. and Wiginton, P.J., 2000. Floodplain Formation and Cottonwood Colonization
Patterns on the Willamette River, Oregon, USA. Environmental Management, 25(1): 87104.
Gutowsky, S., 2000. Riparian Cover Changes Associated with Flow Regulation and Bank
Stabilization along the Upper Willamette River in Oregon between 1939-1996. M.S.
Thesis, Oregon State University, Corvallis, 92 pp.
O'Connor, J.E., Sarna-Wojcicki, A., Wozniak, K.C., Polette, D.J. and Fleck, R.J., 2001. Origin,
extent, and thickness of Quaternary geologic units in Willamette Valley, Oregon. 1620,
U.S. Geological Survey, Reston, Virginia.
O'Connor, J.E., Jones, M.A. and Haluska, T.L., 2003. Floodplain and Channel Dynamics of the
Queets and Quinalt Rivers, Washington, USA. Geomorphology, 51: 31-59.
Wallick, J.R., 2004. Geology, Flooding & Human Activities: Establishing a Hierarchy of
Influence for Controls on Historic Channel Change, Willamette River, Oregon. M.S.
Thesis, Oregon State University.
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