Side channel evolution, geomorphic diversity, and sediment transport on the Bighorn River following larger dam releases between 2008 and 2018

Project ID: 19306
Principal Investigator: Melissa Foster
Research Topic: Sediment Management and River Restoration
Funded Fiscal Years: 2019 and 2020
Keywords: None

Research Question

(1) How do model predictions of sediment transport compare with direct measurements of sediment transport from tracer particles? (2) Are restored side channels sustainable in stream systems with regulated hydrology? (3) How have higher flow releases in recent years affected the geomorphic diversity and stream morphology? (4) Previous studies indicated that the lack of high discharge releases likely led to a loss in geomorphic diversity. However, locals residents and shareholders are concerned such releases have led to an increase in erosion. Is recent erosion significant in terms of historic trends or a part of natural river migration processes (i.e. is recent erosion occurring within historically dynamic areas of the channel morphology)?

The proposed research will test the following hypotheses and conclusions from previous work: (1) the wetted perimeter of the Bighorn River continues to remain relatively stable since dam emplacement, as found in the 2010 study (Godaire, 2010); (2) geomorphic diversity within the channel is likely stable since the previous study and any recent erosion within the channel does not fall outside of historic fluctuations (3) physical excavation is required to reconnect many endangered and disconnected side channels along the Bighorn River, as model predictions indicate that the side channels will not be reconnected with high flows alone (Hilldale, 2012); (4) once restored, high flow releases can help prevent aggradation at side channel entrances.

In addition to site specific questions and hypotheses, the proposed research will provide data that inform model predictions of sediment transport and the effectiveness of restoration projects in a regulated stream system.

Need and Benefit

Reclamation dams have regulated the hydrology and reduced the downstream sediment supply on numerous rivers.
This can lead to a loss in geomorphic diversity on many river systems; for example: (1) side channels can be
disconnected when they are not regularly inundated by high flows, (2) gravel bars can be lost due to a lack of new
sedimentation, (3) vegetation can encroach on sediment that is not regularly inundated or transported. As restoration
projects are often a priority for our Reclamation offices and partners, further study is needed to understand how to
best preserve geomorphic diversity and alluvial river integrity within regulated stream systems.
Trush et al. (2000) suggest that a downscaled morphology could retain a river's diversity and integrity. Essentially, a
stream's hydrograph accomplishes specific geomorphic and ecological functions and the river's morphology (i.e., bars,
floodplains, side channels) should be scaled to what the hydrograph is capable of accomplishing. This raises two
questions: (1) How do we best preserve and restore river morphology on regulated systems? and (2) Can restoration
projects targeting historic morphology be effective or should we target the creation of new, downscaled morphology?
The proposed research contributes data critical to answering this question as side channel evolution is tracked
through decades with changing hydrographs (pre-dam hydrology, post-dam hydrology during drought and high
precipitation cycles which affect releases). In addition, the proposed research provides validation for model results
commonly used to infer sediment movement and inform restoration projects. Numerical models of sediment transport
are lacking in empirical knowledge of bed load transport.
The proposed research will benefit Reclamation by: (1) providing empirical knowledge of sediment transport, which
may be used to inform our numerical models of sediment transport, (2) collecting data to test the effectiveness of a
side channel restoration project at retaining the restored morphology and providing habitat to target species, and (3)
tracking how geomorphic evolution (i.e., erosion, deposition, side channel connections) is either similar or different
following a decade with higher precipitation and discharge releases. In addition, results from this study can be used to
inform reservoir operations.
Restoration projects connecting side channels and floodplains are actively pursued by Reclamation and watershed
groups on the Bighorn River and other regulated stream systems. This project will provide a test case demonstrating
how or if restoration of an endangered or cut-off side channel can be effective with a regulated hydrograph. These
data, and similar projects, will inform how restoration funds could be better spent or better implemented.
In addition, stakeholders along the Bighorn River have expressed concerns that Reclamation's releases from
Yellowtail Dam have contributed to increased erosion. Mapping conducted in this study will allow erosion and
deposition in the last decade to be compared with previous decades to determine if there has been a recent
acceleration in erosion that dramatically differs with that of pervious decades.
Reclamation and other groups actively pursue restoration projects in regulated systems. We need data indicating the
efficacy of restoration projects in regulated systems such as the Bighorn River. In addition, we currently have few
direct measurements of sediment transport to inform our numerical models of sediment transport; these models are
used for both river restoration projects and river management. Local to the project area, Reclamation has received
recent concerns about increased erosion on the Bighorn River; to date, recent erosion has not been documented or
placed within the context of natural river processes and geomorphic change on decadal timescales.

Contributing Partners

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Research Products

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Last Updated: 6/22/20