Substrate / Sediment Conditions

Importance to Salmonids

Gravel provides habitat for newly hatched alevin.

Image courtesy of "Salmon Life Cycle" web page, an educational site for elementary school students.

Salmon and anadromous trout require coarse sediment (gravel) for several stages in the life cycle. Gravel is used in spawning, as habitat for recently emerged fry, and as a substrate for macro-invertebrate prey for juveniles (Flosi et al. 1998; Spence et al. 1996). The quantity and quality of gravel within a stream system are critical to the successful spawning and incubation of salmon. Salmonids require sufficient gravel within a specific size range and a minimum of fine sediment for successful spawning. Clear water, with a limited load of suspended fine sediments (turbidity), also plays a role in feeding efficiency and certain territorial behaviors. Sedimentation rates vary between and within watersheds and are determined by geology, vegetation patterns, and land use.

Human Impacts

Although erosion and sediment production are natural processes, human disturbance has influenced the rate, timing, location, and substrate size of erosion. As rural residential, agricultural, commercial, and industrial development has modified riparian and upslope areas throughout the watersheds in San Mateo and Santa Cruz Counties, the production of fine sediments has increased.

A roadside ditch repaired by the San Mateo Public Works Road Maintenance Division. The ditch now prevents several tons of soil erosion annually.

Photo courtesy of San Mateo Department of Public Works 2004.

The Sand Hills area in Santa Cruz County, for example, is characterized by loosely amalgamated soils that can contribute excessive amounts of fine sands to the in-stream environment in response to disturbance events. Riparian buffers – efficient at trapping fine sediments prior to delivery into the stream channel – have been removed or degraded in many areas. The removal of upslope vegetation for agriculture or rural residential purposes may increase the volume and delivery rates of fine sediments to aquatic systems. Dams may intercept coarse sediments, reducing gravel delivery to downstream areas. Roads, especially unpaved roads, are a major source of sediment. Roads are a special concern because they are a source of erosion (both mass wasting and long-term surface erosion) and a supply route. Road surfaces provide a constant supply of fine sediment and a mechanism to move sediment during storm events. Sediment is transported along the road surface into storm drains and culverts and is finally deposited into creeks and streams. Roads alter the natural drainage system, causing changes in surface and subsurface water flow as well as increasing the impervious surface area. Control of water movement and drainage requires use of road support structures, such as road cuts, ditches, and drainage culverts, but, if poorly engineered, these structures can create new sediment sources, increase surface water flow, and provide sediment transport mechanisms. Road crossings at streams and stream culverts are potential high sediment dangers because of mechanical failure ("wash-out") during storm events. Roads built on unstable slopes or poorly engineered and maintained roads can trigger landslides and mass wasting, resulting in large sediment inputs, changes in stream course, and long-term erosion problems if left unmitigated.

References

Flosi, G., S. Downie, J. Hopelain, M. Bird, R. Coey, and B. Collins. 1998. "California Salmonid Stream Restoration Manual. Third Edition." State of California Resources Agency, California Department of Fish and Game, Inland Fisheries Division. 495 pp. View on-line source.

San Mateo County Department of Public Works. 2004. Watershed Projects and Programs [Web page] [cited 2004]. View on-line source.

Spence, B.C., G.A. Lomnicky, R.M. Hughes, and R.P. Novitski. 1996. "An Ecosystem Approach to Salmonid Conservation." ManTech Environmental Research Services Corp. TR-4501-96-6057.

Local References

Chartrand, S., B. Hecht, D. Alley, and T. Danzig. 2002. "Arana Gulch Watershed Enhancement Plan Phase 1: Steelhead and Sediment Assessments, Santa Cruz County, California." Balance Hydrologics, Inc., D. W. Alley & Associates, Coastal Watershed Council. Prepared for Arana Gulch Watershed Alliance with funding from California Coastal Conservancy and California Department of Fish and Game, 115 pp.

Conrad, M.T. 2003. "Gazos Creek Watershed Assessment and Enhancement Plan: Appendix D: Road and Land-Use Survey of Gazos Creek Watershed." Prepared for Coastal Watershed Council. 17 pp. View on-line source.

Scotts Creek Watershed Council. 2000. "Road and Landslide Sediment Source Investigation and Sediment Reduction Plan for the Scotts Creek Watershed, Santa Cruz County, California."

Swanson Hydrology and Geomorphology. 2001. "Zayante Area Sediment Source Study."

Swanson Hydrology and Geomorphology. 2001. "Technical Addendum to Zayante Area Sediment Source Study."

California State Water Resources Control Board. 2003. 303d Listings for Sediment on Pescadero and Butano, San Francisquito Creek, San Gregorio Creek, San Lorenzo Creek, Soquel Lagoon, Aptos Creek, Pajaro River [Web site]. SWRCB [cited 2003]. View on-line source.

General References

Weaver, W. E., and D.K. Hagans. 1994. "Handbook for Forest and Ranch Roads." Pacific Watershed Associates. 161 pp.

NOAA National Marine Fisheries Service (NMFS). 2000. "Guidelines for Salmonid Passage at Stream Crossings." NMFS Southwest Regional Office.