Assessment of Habitat Complexity and Ecological Functions Provided by Gravel Bars Resulting from Gravel Augmentation and Channel Rehabilitation

Project ID: 5907
Principal Investigator: David Gaeuman
Research Topic: Sediment Management and River Restoration
Funded Fiscal Years: 2013
Keywords: gravel bars, habitat complexity, ecosystem services, sediment management, river restoration

Research Question

The Trinity River Restoration Program (TRRP) has been at the forefront of efforts to restore coarse sediment supply and transport in regulated rivers, with the goal of recreating habitat complexity lost due to sediment starvation by upstream dams. New gravel bars have formed through natural deposition downstream of augmentation sites and have been constructed through channel rehabilitation and side channel restoration. While the physical processes involved in augmentation of coarse sediment, rehabilitating fossilized channels, and resulting channel changes continue to be well-documented through fluvial geomorphic research, there has been little empirical research to document the contribution of restored gravel bars to ecological functions (besides direct measurements of salmon habitats such as spawning gravels). The purpose of the proposed research is to document the degree to which gravel bars created by restoration actions contribute to ecological functions, notably through the diversity of habitats needed by aquatic macroinvertebrates and the natural filter effect of gravel bars (shown elsewhere to play a significant role in retention of organic matter in dam tailwater ecosystems). The proposed research is also designed to provide insights into the role of different mechanisms of gravel bar restoration and the influence of different gravel mixtures (clean/washed gravel without fines vs pit-run gravels containing up to 20% sand) on resulting microhabitat, hyporheic exchange, and organic matter retention. Results of the proposed research will provide quantitative measures of the ecological benefits of the restoration program and will inform future restoration planning in terms of overall restoration approaches and materials used. The proposed research represents an opportunity to build upon existing work by adding new ecological dimensions to physical-process research already underway.

Need and Benefit

Reclamation has supported development of river restoration strategies and related techniques to restore riverine ecosystem services degraded by impacts of large dams and water diversions. In particular, California river restoration programs have needed to restore diverse habitat structures required for anadromous salmonids in their river life stages, including adult holding (adequate space and depth) and spawning (fresh gravel with adequate subgravel flow), egg incubation (adequate water temperature and dissolved oxygen), fry rearing (low velocity and shallow habitat), juvenile rearing (faster and deeper habitat for growth and feeding) and smolt outmigration. Also, maintaining quantity and quality of riverine organic matters and macroinvertebrates healthy is fundamental for the salmonids as food resources diversity, as well as river ecosystems.

Unfortunately, gravel bar features below dams have been lost by reduced coarse sediment transport downstream (Kondolf 1997, Petts & Gurnell 2005). To date, TRRP has created gravel bars through deposition downstream of gravel augmentation sites and through channel rehabilitation activities. Guided by two decades of fundamental and applied research, the TRRP has been at the forefront of restoring physical processes to rivers downstream of dams. While gravel augmentation efforts have been undertaken in many rivers in North America, Europe, and Asia, few can compare to the scale and ecological ambition of the Trinity River program. However, the ecological roles of the newly formed geomorphologic features have been yet to be assessed from the critical perspectives of habitat diversity, macro-invertebrate diversity, and organic matter interactions. Improved understanding of the ecological role of gravel bars could yield insights to inform environmental management of gravel bar design, implementation of rehabilitation activity, and maintenance.

The most immediate application of the research results will be to the Trinity River, but the approach and some of the insights should be applicable to the Elwha River (where removal of two dams will greatly increase sediment load downstream and will likely lead to bar formation). The principles developed in this study can potentially be applied to other rivers for which Reclamation has responsibility, including the Rio Grande, the rivers of the Sacramento River system.

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.

Assessing Gravel Bars Habitat Complexity and Ecological Functions (final, PDF, 390KB)
By David Gaeuman
Publication completed on September 30, 2013

This bulletin summarizes the research results and potential application to Reclamation's mission.

Not Reviewed

The following documents were not reviewed. Statements made in these documents are those of the authors. The findings have not been verified.

Assessment of Habitat Complexity and Ecological Functions provided by Gravel Bars resulting from Gravel Augmentation and Channel Rehabilitation in the Trinity River (final, PDF, 93KB)
By Giyoung Ock and Dr. G. Mathias Kondolf
Report completed on June 30, 2014

Restored gravel bars provides simplified regulated reaches with hydraulic diversity in velocity and depth, thermal diversity via hyporheic flow, food source diversity through S-POM retention and B-POM distribution (depending on trophic source types), and bed substratum diversity via upwelling hyporheic flow. Restored gravel bars enhance habitat complexity, which supports salmonid recovery, as well as a wide range of other life forms.

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Last Updated: 6/22/20