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Predicting the Interactions between Flow, Sediment, and Riparian Vegetation

Project ID: 1368
Principal Investigator: Blair Greimann
Research Topic: Sediment Management and River Restoration
Priority Area Assignments: 2011 (Place Holder - Do not use), 2012 (Place Holder - Do not use)
Funded Fiscal Years: 2011, 2012 and 2013
Keywords: None

Research Question

What impact does riparian vegetation have on local flood conditions (water surface elevation)?

How can vegetation be incorporated into restoration projects without increasing flood risks?

What set of reservoir operations can be used to encourage successful native vegetation recruitment and survival? Can reservoir operations be used to control invasive species?

Need and Benefit

Today, it is widely recognized that past engineering and economic water resources development policies, while often meeting specific economic objectives, tended to degrade natural ecosystems. This degraded condition, combined with an increased appreciation for the beneficial services provided by properly functioning ecosystems, has prompted efforts in many basins to restore rivers to a more natural state through physical improvements or modifications to flow management. There are several such examples of this at the Bureau of Reclamation (Reclamation).

On the Platte River, it was recognized that low flows and reduced sediment supply have caused the channel to narrow and vegetation to encroach. A restoration project is underway to improve avian habitat that was reduced due to this narrowing. A successful and sustainable restoration project must optimize the flow and sediment management to maintain a wider channel with reduced vegetation encroachment.

In some reaches of the San Joaquin River, the flow was essentially stopped for the last several decades, causing desiccation of most all native riparian vegetation. The San Joaquin River Restoration project has the goal of returning salmon to the river, and riparian vegetation will be critical to the success of the project. It will provide juvenile habitat and reduce water temperatures. However, the flood control system has been built and maintained under the assumption that little riparian vegetation would be allowed to exist in the reaches with levees. Therefore, the project must strike a balance between encouraging vegetation for habitat purposes and limiting vegetation for flood control purposes.

On the Sacramento River, much of the once cottonwood dominated riparian forest has been replaced by agriculture. Also, the altered flow regime is less favorable to cottonwood survival and recruitment due to storage of the once plentiful spring runoff flows in Shasta and other reservoirs. Also, extensive bank stabilization and levees along the Sacramento have limited river meandering, which is critical to the regeneration of cottonwood forests. Without active river meandering, new cottonwood growth is severely limited because cottonwoods generally require bare substrate to establish. These factors have decreased the extent of the riparian forest to a fraction of their historical value.

Existing engineering methods and tools are in many ways inadequate. They are generally limited by a lack of quantitative relationships between the effect of flow management on riparian vegetation and the effect of riparian vegetation on flood management. For example, hydraulic models are used to assess flood risk, but such models do not adequately represent the processes of vegetation and flow field interactions. Thus, restoration of riparian vegetation introduces significant uncertainty into estimates of water surface elevations within flood studies. As another example, riparian recruitment flows are necessary to encourage sustainable diverse riparian forests, which are critical to successful restoration projects. However, there is limited quantitative information available as to how much flow is necessary.

The current proposal is an ambitious one--to develop tools to predict the interactions of flow, sediment, and vegetation. These sets of tools would help quantify several Reclamation actions for which we currently do not have quantitative methods. We will have better understanding of how flow operations affect riparian vegetation downstream of our facilities. We will be able to predict the impact increased riparian vegetation may have on the water surface elevations during flood events. We will be able to determine which set of flow operations will be most beneficial for native riparian vegetation.

Contributing Partners


Research Products

Bureau of Reclamation Review

The following documents were reviewed by experts in fields relating to this project's study and findings. The results were determined to be achieved using valid means.

Calibration of Numerical Models for the Simulation of Sediment Transport, River Migration, and Vegetation Growth on the Sacramento River, California (interim, PDF, 6.0MB)
By Dr. Lisa Fotherby, Victor Huang, Dr. Yong Lai, Blair Greimann and Dr. Charles Young
Report completed on March 27, 2011

This report describes the calibration and/or application of five numerical models that together simulate processes of flow hydraulics, sediment transport, river meandering, and the establishment and survival of riparian vegetation in the Sacramento River Corridor in California. The study area extends from Red Bluff Diversion Dam to Colusa. Results of this modeling effort will be used to analyze the effects of project operations on the Sacramento River for the Bureau of Reclamation.
Keywords: cottonwood, river meandering, sediment transport

This information was last updated on April 19, 2014
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