May 10, 2010
Dear Mr. Hines,
Thank you for asking about a ‘sediment budget’ in our recent meeting regarding the feasibility study for
recreating floodplain landforms out of the gravel pits along the Middle Reach of the Russian River.
Sediment supply is the core issue in modeling the feasibility of reshaping and connecting the pits to the
river. I understand your question and inquiry come from a long-standing interest in the larger issue of
sediment resource management throughout the Russian River watershed. So, I wanted to respond
from that perspective. That way, our feasibility study and the planned USGS modeling of the study
reach (from Healdsburg to Wohler Narrows) is explained within that larger scientific, spatial, and
resource management context.
I’m sure you are aware that just five decades ago the Russian River’s Middle Reach channel was
relatively shallow, meandering and in places multi-threaded. From early air photos we can see scars
from its channel winding across the broad floodplain. The Middle Reach valley floodplain functioned as
a deposition zone where the river stored its sediment inputs from the upper reaches when transported
on the large flood events. Over time common winter flows would have reworked those deposits and
selectively transferred portions of them downstream over a longer time-scale, metering the sediment
supply to the confined reach downstream, and to the ocean. This buffering effect was important for
deep pool formation and resilience, and also dampened flood stage elevations along the downstream
reaches to the ocean.
The early settlers began modifying the Russian River valleys almost 2 centuries ago, but starting in the
1950’s, channel dredging and dam-associated ‘flood control’ projects (bank revetments and levees) and
the incision they caused are what facilitated encroachment by agriculture, and then isolation of
floodplain that was subsequently targeted for “terrace” gravel mining pits. That dredging of the
channel, incision and leveeing, fundamentally changed the sediment transport regime, the fluvial
geomorphic dynamics, and the multi-seasonal aquatic habitat conditions in the Middle Reach. These
changes in physical conditions and processes also altered, and had a tremendous impact on the
downstream evolution of the channel form, multi-seasonal aquatic habitat conditions, and flood
elevations.
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In its present configuration, the Middle Reach channel has never been straighter or steeper (50%
increase between 1940 and 1993), more incised (doubled between 1940 and 1993), hydraulically
smoother, and more capable of transporting right through the reach most of the sediment supplied
from upstream. [Sediment transport capacity is the product of depth multiplied by slope, and a couple
of constants]. In effect, the current river squanders the sediment supplied to it from upstream. This is
hard to observe even when you know it occurs, and what we see after a flood passes is that gravel bars
build up in the Middle Reach. Bars grow to an optimal size and form depending on the hydraulic field of
the flood flows that allow localized deposition during considerable bedload movement events.
The reasons these bars have been repeatedly mined include maintaining ‘flood capacity’ of the channel
with minimal to no overbank flow or detention of flood waters, and to maintain the incised channel’s
sediment transport capacity through the reach. It is really important to understand that those bar
deposits represent only a fraction of the bedload sediment transported through the Reach on the large
events. The Middle Reach as currently modified is extremely capable of transporting sediment, and
unfortunately from a habitat forming perspective, very, very inefficient at storing sediment.
This history of valley floodplain modification is not unique to the Middle Reach. Every tributary valley in
the Russian River basin has a history of elimination of wetlands, lowered water table through base level
lowering and channelization, and encroachment (often including levees), that transport sediment
through valley reaches (i.e. flood control projects). All of the channels of the tributary valleys are
incised, and their channels disconnected from floodplains. Across the entire Russian River Basin, every
natural sediment deposition area has been cut off from performing that function, and from providing
aquatic habitat. We see the results in the status of the listed salmonid species (of which there used to
be five).
As you point out there are two large Dams (Coyote and Warm Springs) in the basin that trap bed load
sediment, but from only 13% of the watershed. While significant near the dams, this percentage does
not control sediment supply at points downstream. The small dams you mention, numerous for sure,
are in mostly one of two states; filled with sediment long ago and therefore not trapping any more, or
seasonally closed and opened in winter, and not trapping sediment either. The biggest problem with
sediment management in the basin is the ubiquitous conversion of natural deposition zones (alluvial
valleys) into sediment transfer reaches. Ironically, the practice of bank stabilization in the valleys
insures continued channelization and squandered sediment resources through the valley reaches.
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Recent advances in fish tracking technology show that ephemeral winter upland wetlands are key to
rapid winter growth opportunities for coho. Studies have documented coho mean densities up to 12
times greater, and growth rates up to 49% faster, in off channel habitat than in main channel habitats.
The completely channelized network of streams throughout the Russian River Basin simply no longer
support a wetland adapted species like coho.
So you are very correct in identifying sediment supply as the central component in determining the
feasible alternatives for reforming the gravel pits and connecting the river to what will hopefully
become a restored deposition zone. As you point out, sediment supply is central to the understanding
of how a river will respond to changes in topography, and for predicting changes in local deposition and
transport patterns (and erosion or stabilization of incised banks).
Sediment budget background and context
Basin-wide sediment budgets are a tool for organizing and scaling how to think about how a river system
processes sediment within its basin. Sediment budgets are useful for helping us scale, relatively, the
different processes, and can provide insight for what we should monitor when basin landforms or river
system stream networks are disturbed and altered for social purposes. In an undisturbed system, a
sediment budget would be very useful for planning the best locations to mine aggregate, as well as for
determining the most sensitive aquatic habitat areas where conserving natural sediment transport
processes is critical to maintaining the seasonal set of conditions that support thriving salmonid life
history stages population abundance. If only we could rewind the clock and plan for holistic river
management!!
Sediment budgets integrate large geographic areas and average movement (transport) over decadal
time scales, and express the results in annual averages. An average is just too crude of a measure to
manage site specific resource extraction like gravel bar mining. Using a traditional sediment budget
approach, a mined out bar can remain idle for years and then be redeposited in days during large storm
flows with massive bedload transport. In our El Nino climate, it is deposition and erosion that
determines how a river’s channel and floodplain form.
So the analogy of a “sediment budget” to sediment supply, and the use of that idea and concept by
many ‘restoration’ practitioners is convenient, but is actually somewhat misleading in that there is great
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variability in the quantity of both incoming sediment, and that which is transported out of any given
reach. And that is particularly applicable in our El Nino driven climate.
In many ways sediment budgets bring up political rather than scientific debate. Scientists understand
the complexities and reasons why the results can be taken as precise and accurate, when they are not
even determinate given the range that should be reported. Nonetheless, sediment budget retains some
attractiveness for the management of gravel mining in some locations. It is rooted in the concept that
rivers deliver an annual supply of sediment and if too much of that supply is removed there will be
erosion downstream. As a concept, that is sensible, but it gets very complicated very quickly when we
look into any of the details. First a sediment budget does not give accurate results, it gives a range, and
that range is typically an order of magnitude (factor of 10) or more in size. This is because all of the
terms in the equation are ranges, not single values. The supply term is inherently variable (geology,
climate, land use; the latter two constantly changing), and better spatial data does not appreciably
reduce the data range, especially over time. Then, the export term is subject to several physical
processes that each operates over very large ranges. Not the least of which is flow.
Flow is driven by climate and here in Northern California, the climate cycles of El Nino oscillate between
drought lasting 4 years or more and flood every 5th year, on average. And as you know having lived here
for many years, some El Nino’s hardly register. So large sediment delivery and export processes don’t
occur very often; they are decadal processes, not annual.
Sediment transport (the export term) is also driven by several physical processes that operate over wide
ranges, but one physical process that is particularly confounding is the threshold of bedload
mobilization. Stream bed sediment made up of gravel and cobble does not mobilize until relatively high
flows occur, which are infrequent and typically brief. And to further confound coarse sediment
transport from being a simple to predict process, a cobble armor layer forms naturally on streams beds.
The cobble layer overlying gravel and sand deposited on previous events inhibits the mobilization of the
underlying sediments until the force of flow is great enough to mobilize the cobble. So gravel bars and
any armored part of a stream bed only reach threshold of bedload mobilization during the large and
infrequent El Nino generated storms.
Defining an annual sediment budget is therefore a nominal process, and that has to be understood
when applying sediment budget concepts to resource management at a site or reach scale. It can be
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argued that the error bars in a sediment budget are more informative for planning decisions than the
average estimates are.
There are several quantities in the three terms on each side of the equation for sediment accumulation
in a defined reach (sediment transport quantity in – (minus) sediment transport quantity out = change in
sediment storage quantity) (“gravel bars”). Each of these variables has a range; they are not
deterministic values. They are debated by experts, not fully appreciated by many ‘restoration’
practitioners, and are just flat out misused by some consultants.
This simple equation, unfortunately, is not as simple as balancing a checking account because, (1) there
are not regular deposits – most coarse sediment is delivered by infrequent El Nino floods, (2) the units
of deposit and withdrawal change currency - grain size matters a lot as they are sorted, abraded, and
transported differentially, and (3) the account “reserves” cannot be determined (or agreed upon
because they are an order or two of magnitude in range). So in considering how a sediment budget
relates to the physical processes in a particular river reach, it is really important that we view the bars of
stored sediment in a reach as representing but a fraction of the flux of sediment through that reach.
When applied to gravel mining, sediment budgets get really tricky. In theory, of course, mining within
the ‘budget’ conserves geomorphic processes and multi-seasonal aquatic habitat conditions. [The
percentage of budget required to maintain geomorphic features and habitat is not known!!]. While in
practice, bars that create habitat are not formed or replaced during annual storm events. Managing
mining with sediment budget has not worked very well on the Mad River where entire bars are mined to
the thalweg every year, “within budget”, but with little attention to conserving the form and function of
bars and habitat. The sediment budget approach to managing mining also sets up an expectation (by
industry) that the annual quantity can be mined every year and the river will not be harmed. This is
certainly not the case, especially where El Nino climate cycles dominate sediment transport events.
The other common method to manage mining is the redline method. When modified to include
geomorphic features, as in Sonoma County, this method focuses on conserving geomorphic features and
habitat and does it with objectives and monitoring that adapt to climate cycles and unforeseen events,
just as the sediment budget concept, if fully understood, suggests. The sediment storage term is the
important one when modifying geomorphic features; after all, bars are sediment in temporary storage
till the next El Nino event. Maintaining bar form throughout normal climate and drought periods is what
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conserves habitat until the subsequent El Nino bedload mobilization replaces sediment mined from a
bar. Compare that with mining regulated by maintaining a “sediment budget” and ignoring bar form,
which ensures maximum impact to habitat sustaining processes during the prolonged normal water year
and drought cycles.
Another example of misuse of the sediment budget concept is for total maximum daily load (TMDL)
management, where TMDL is concerned with fine sediment produced from large geographic areas. Fine
sediment is the sizes that easily stay in suspension in turbulent flowing water and cause turbidity; sand
and smaller. Ironically, management of sediment from the perspective that it is a pollutant has led to a
lot of questionable repairing of eroding banks, which in many cases is actually counterproductive to the
need for coarse sediment supply from both upstream and the banks to refill the incised stream
channels, and to restore the health of habitat.
Thus, we, as restoration practitioners have to be very specific (geographically, temporally, and by
sediment grain size) in defining the term sediment budget. Among the points I want to make here is
that not all sediment sources are the same grain size, not all visible sediment is available as a source, not
all reaches of stream transport sediment with the same efficiency (some reaches need sediment load or
they incise), and sediment budgets are not capable of discriminating these differences with accuracy
(but you will see results reported with great precision!!!). Consequently sediment budgets are
considered academically to be useful for planning and not recommended for project design, even
though many consultants make a living in the latter (unfortunately for the resource).
Sediment budget in the context of the proposed gravel pit reconnection project
With the above background on sediment budget, the pit reconnection feasibility study needs to develop
a sediment input to the model reach (i.e., the upstream boundary condition, in model jargon). This is
much the same problem that sediment modeling deals with in dam removal studies, where there are
usually not many specific sediment transport measurements to use as model inputs. This problem is
dealt with in a couple of different ways but the most common is a two-step approach of first estimating
a ratio of sediment load for a range of flows (called a sediment rating curve) using watershed geology,
hydrology, geomorphology, and land use (like a sediment budget would), and then actually testing how
that model performs by running various flood scenarios that have occurred in the gauged record to see
how well the model replicates how the river channel morphology indeed changed during those floods.
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This second step is called model equilibration. The equilibration process involves adjusting the
upstream sediment input equation until the model reasonably replicates the river’s history of
adjustments during various floods. In the case of the Middle Reach, accounting for the history of gravel
mining in the river will be an important part of model equilibration.
For this project, we need to determine sediment load at a specific location, ie, the upstream model
boundary, which will be at Healdsburg. A sediment supply rating curve, a relationship between river
flow and sediment transport, will be developed for the model input at the upstream boundary. The
rating curve will be representative of the grain sizes in transport and related to stream flow, not just a
lump sum of all sediment eroded from the watershed upstream. The model will route that sediment,
scour and fill within the model confines, and examine the output (sediment load) and the effects of that
load at the downstream boundary (Wohler Narrows). A basin- wide sediment budget would inform this
process if one were available, but we'd still have to develop and verify a sediment rating curve for the
model input. The sediment rating curve will be developed using the information available and that will
include estimating sediment delivery from the basin (watershed), examining all sediment transport
measurements from the basin and from similar geologic/climate settings in the region, measuring the
sediment properties in the channel and throughout the project area, calculating the force of flowing
water required to mobilize the existing bed and project exposures, reviewing the literature on sediment
transport relationships that match up with this setting, and then a thorough model equilibration process
that accounts for the history of channel modifications and gravel extractions. And then, as
conscientious modelers do, we will examine the sensitivity of the inputs and transport parameters to
report a probable range of outcomes within the capabilities of the model, and not defend precise but
inaccurate model results. The most important concern about the proposed project is assessing whether
restoring the Middle Reach to a deposition zone will starve the downstream of sediment (specific to
grain sizes that matter for channel stability) and cause erosion -so sediment supply and how the project
processes the supply is the key.
I think the modeling we are setting out to conduct with the USGS will be an exciting contribution to river
management in the Russian River, and it has broader implications for river management throughout the
basin and many places at home and abroad. It will predict how the river sediment entering the project
reach will be deposited and transferred, and what the eventual topography will look like. It will predict
the sediment load exiting the project reach, and it will predict the scour or fill in the reach downstream.
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It will also predict changes in flood elevation and it will be the basis for biological assessments of the
new landscape and multi-seasonal aquatic habitat conditions for fisheries and other wildlife, and the
formation of a terrestrial wetland riparian complex. And it will allow us a very informed look into the
future of the Middle Reach (whether we implement the project or not) as the proposed project area
evolves with sediment deposition and formation of new channel and floodplain features. I think the
modeling we propose for the project feasibility will thoroughly answer the questions you raise about
sediment for the Middle Reach. Scaling it up to help understand and manage other reaches is certainly
possible, and perhaps this specific restoration project will prompt a basin-wide effort.
I know I speak for the entire project team that we look forward to working through these details of the
modeling effort and sharing them with you and the stakeholders as part of the process. Please let me
know if you have more questions, by email (brian.cluer@noaa.gov) or phone (707-575-6061). Perhaps
we can discuss this further, if needed, at a future meeting.
Sincerely, Brian Cluer
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