See also: Flow Regimes as a Limiting Factor

Addressing Sediment as a Limiting Factor

Restoring riverine habitats to conditions favorable for supporting salmonids through their reproductive and smoltification life stages as adults, eggs, and juveniles is critical to our necessary response for recovering aquatic biodiversity. The acceleration of fine-sediment delivery and storage in coastal rivers and inland tributaries supporting salmonids drastically decreases egg survival, foraging success, and juvenile growth while increasing injury.  This problem is extreme in northern California where about 59% of watersheds are impaired by sediment . The literature is full of excellent studies that experimentally and empirically characterize the detrimental impacts of fine sediment of salmonids, and some of these are further highlighted here.

In 1985, Berg and Northcote , experimentally demonstrated changes in behavior of juvenile coho salmon to short-term pulses of suspended sediment.  During these exposure tests, individual fish left their territories and had irritated gills.  Additionally, reaction distances to prey, capture success rates, and prey ingestion rates declined during periods of higher turbidity. More recently, Lake and Hinch described that the type of fine sediment was linked to fish stress and mortality in juvenile coho salmon. In an experimental comparison of distinct types of suspended sediment, natural fluvial sediments caused fish stress and mortality at much lower concentrations than found with anthropocentrically derived “extremely angular” suspended sediment. The behavioral modifications of juvenile coho salmon during a critical period may limit recovery opportunities where watershed condition is limited by fine sediment. Fine sediment enters streams from numerous pathways, and should be managed within and beyond the riparian zone.

Sediment also directly impacts the aquatic food chain in numerous streams creating habitats less favorable to salmonid survival A recent study completed at UC Berkeley found an increase in fine sediment deposition shifted the macroinvertebrate community towards burrowing taxa, which are less available for prey, and increased metabolic costs in fishes associated with greater activity and intraspecific competition.  A linear response between increased deposited fine sediment and decreased juvenile steelhead growth suggests there is no threshold below which increased fine sediment delievery and storage is harmless, and the impact of sediment on stream food webs directly reduces salmonids ability to grow and survive. This study also suggested that any reduction in fine sediment could produce immediate benefits for salmonid restoration. 

Embeddedness is a measure of fine sediment in spawning gravel. It is a common metric used by the Regional Water Quality Boards and National Marine Fisheries Services to identify sediment as a limiting factor in restoring spawning gravels.  The desired condition for a recovering watershed to support greater salmonid spawning habitat is found where there is an increasing trend of locations where gravel and cobbles are ≤ 25% embedded.  While emphasis is often placed on reducing direct inputs of sediment into streams, Opperman et al. found that coarse-scale quantification of watershed land was significantly related to measurement of embeddedness. In watersheds with agricultural and urban footprints, often characterized by restricted riparian areas, there was often not a direct decrease in fine sediment and reduction of sediment is best accomplished by envisioning reduction throughout the entire watershed. While reach-specific riparian protection offers numerous benefits to salmonids via cover, nutrient, temperature buffer, this type of protection has limited benefits for overall reduction of sediment.     

Many restoration projects offer multiple benefits to salmonids during their various freshwater life history stages and holistic projects managing sediment from indirect and direct sources is critical in restoring salmon to viable population sizes in may coastal streams and inland rivers.  Properly functioning roads, stormwater systems, and sufficient riparian buffers are necessary in many watersheds to adequately reduce fine sediment entering streams. SRF continues to highlight actions to reduce sediment using best management practices for Road upgrading and instream bioengineering projects, and will feature a number of sessions centered on water quality, water quantity and salmonids at the upcoming 2006 conference. If you are more interested in quantifying, monitoring, and assessing fine sediment in relationship to salmonids a number of great agency resources exist including:

Flosi, G., S. Downie, J. Hopelain, M. Bird, R. Coey, and B. Collins, 2004. Updated California Salmonid Stream Habitat Restoration Manual. 3rd Ed. Department of Fish and Game, Inland Fisheries Division.

NOAA Fisheries- Southwest Region. 2004. Sediment removal from freshwater salmonid habitat: guidelines to NOAA Fisheries staff for the evaluation of sediment removal actions from California streams.

North Coast Regional Water Quality Control Board. 2006. Desired salmonid freshwater habitat conditions for sediment-related indices. Available via the water board’s Basin Plan TMDL Implementation website http://www.waterboards.ca.gov/northcoast/programs/basinplan/tipfsiw.html

If you would like to share additional resources about sediment and salmonids, please contact Josh at joshuais@sbcglobal.net.  Look for the next SRF newsletter to include a short essay on stream flow as a limiting factor.