Research and Development Office

Effects of Climate Change and Reservoir Operations on Riparian Vegetation

Project ID: 1596
Principal Investigator: Blair Greimann
Research Topic: Sediment Management and River Restoration
Priority Area Assignments: 2014 (Climate Change and Variability Research), 2015 (Climate Change and Variability Research), 2016 (Climate Change and Variability Research)
Funded Fiscal Years: 2014, 2015 and 2016
Keywords: riparian vegetation, reservoir operations, river hydraulics

Research Question

The basic questions this proposal will answer are:

1. What is the potential range of effects of future climate changes on the establishment and sustainability of riparian forests in managed river systems?

2. What set of reservoir operations can be used to encourage successful native vegetation recruitment and survival under projected climate changes?

3. Can reservoir operations be used to control other potential indirect effects such as invasive species?

The Science and Technology Program (S&T) in conjunction with other federal agencies identified knowledge gaps and research priorities for climate change assessment and adaptation planning (Reclamation, 2011). This proposal addresses Gap Number 4.07: "Understanding on how climate change may impact riparian ecosystems and vegetation that affect both longer-term water budgets and ecological resources."

Studies to improve management of riparian ecosystems in Reclamation Projects would be greatly strengthened through application of predictive tools concerning the effects of projected climatic changes and riparian vegetation and sediment processes. Previous research in FY2011 through 2013 funded by the Reclamation Science and Technology program developed computer models that link the establishment, growth, and mortality of riparian vegetation to the hydraulic conditions within the river. This was done by developing vegetation computer modules that integrate with the hydraulic and sediment transport computer models SRH-2D and SRH-1D. These models have been used throughout Reclamation to analyze hydraulic and sediment transport problems on a large number of projects.

The goal of this proposal is to increase the capabilities of the vegetation modeling to include:

1. The simulation of the effect of climate on riparian plant growth
2. The simulation of the effect of riparian plants on sediment processes

Need and Benefit

The need for a quantitative understanding of riparian processes is summarized in Perry et al. (2012): "Together, climate change and climate-driven changes in streamflow are likely to reduce abundance of dominant, native, early-successional tree species, favor herbaceous species and both drought-tolerant and late-successional woody species (including many introduced species), reduce habitat quality for many riparian animals, and slow litter decomposition and nutrient cycling. Climate-driven changes in human water demand and associated water management may intensify these effects. On some regulated rivers, however, reservoir releases could be managed to protect riparian ecosystem."

On the Platte River, low flows and a reduced sediment supply were recognized as causes of channel narrowing and vegetation encroachment. In a rare occurrence, river restoration in this application takes the form of management actions to reduce the establishment of vegetation. In some reaches of the San Joaquin River, most of the flow has been diverted in recent decades, causing desiccation of a significant portion of the native riparian vegetation. The goal of the San Joaquin River Restoration project is to return salmon migrations to the river and a return of riparian vegetation to these formerly dry sections of river will be critical to the success of the project. On the Sacramento River, much of the cottonwood dominated riparian forest has been replaced by agriculture and the extent of the riparian forests has been reduced to a fraction of their historical value. Two of the causes for this change are an altered flow regime and limitations on river meandering.

Studies to improve the management of riparian ecosystems in all of the above cases would be greatly strengthened through application of predictive tools concerning the effects of (1) projected climatic changes and (2) riparian vegetation and sediment processes. Previous research in FY2011 through 2013 funded by the Reclamation Science and Technology program developed computer models that link the establishment, growth, and mortality of riparian vegetation to the hydraulic conditions within the river. This was done by developing vegetation computer modules that integrate with the hydraulic and sediment transport computer models SRH-2D and SRH-1D. These models have been used throughout Reclamation to analyze hydraulic and sediment transport problems on a large number of projects including the Klamath Dam Removal Studies, Rio Grande, Trinity River Restoration Program, North of Delta Off-Stream Storage Investigations , San Joaquin River Restoration Program, Columbia/Snake/Salmon Recovery Program, Yakima River Basin Water Enhancement Program, and Glen Canyon Dam Adaptive Management Program.

In addition to wide use within Reclamation, both SRH-1D and SRH-2D receive support from other federal agencies to further their development. In FY2013, Federal Highways Administration (FHA) funded the integration of SRH-2D into SMS, which is the standard pre- and post-processor package used in complex hydraulic studies performed by the FHA. The US Army Engineering Research and Development Center (ERDC) approved the pre-proposal for a project between the ERDC and the TSC to integrate the vegetation modeling capabilities of SRH-1D into HEC-RAS. HEC-RAS is the standard hydraulic and sediment transport model for all US federal agencies and for private industry.

Integrating the simulation of climate effects on streamflow and riparian vegetation into SRH-1D and SRH-2D will create the opportunity to simultaneously analyze the effects of both climate change and reservoir operations on riparian plant communities. Existing engineering methods and tools are in many ways inadequate for informing such efforts because they do not simultaneously consider river geometry, river flows, sediment transport, vegetation response, and the interrelationships between these factors.

Contributing Partners

Contact the Principal Investigator for information about 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.

Effects of Climate Change and Reservoir Operations on Riparian Vegetation (final, PDF, 527KB)
By Blair Greimann
Publication completed on September 30, 2016

Riparian Vegetation: Reclamation's Ally in River Restoration (final, PDF, 544KB)
By Blair Greimann
Publication completed on March 01, 2017

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