Many rivers in the United States have been disturbed by increases in sediment loads from activities such as mining, grazing, agriculture, logging, and road construction. Since 1955, in Redwood Creek, north coastal California, several large floods resulted in widespread landsliding and gullying from previously logged hillslopes, as well as channel sedimentation and burial of streamside redwood trees. Because of the impact of this excess sedimentation on salmon habitat, Redwood Creek is currently listed as “sediment impaired” under the Federal Clean Water Act.
To track the fate of fresh flood deposits in the Redwood Creek basin, the U. S. Geological Survey (USGS) and the National Park Service (NPS) established a channel monitoring program in the mid-1970’s. Since then, they have been measuring river channel changes through field surveys of channel width and streambank erosion, river bed elevation, number and depth of pools, and analyses of the gravel particle size on the river bed.
The history of the flood sediments differs between upstream and downstream reaches of the river. Channel aggradation (the filling in of a channel bed with sediment) was especially severe in the headwaters of the Redwood Creek basin, but by the mid-1980’s the river had cut down to pre-flood levels, the size of gravel in the streambed had doubled, and remnants of flood deposits along valley walls were becoming well vegetated and stabilized. In contrast, in the downstream portion of Redwood Creek, channel aggradation (filling) continued through the 1980’s as sediment was flushed from upstream to downstream. Here, the channel bed is still in the process of downcutting, and the size of streambed gravel is slowly increasing. This process is illustrated in Figure 1, where the river is cutting through sandy flood deposits down to a gravel and boulder river bed.
As the river has cut through previous flood deposits, pool frequencies and depths have increased. More pools exist in the upstream reach, which has been recovering for a longer time (30 years) than in a reach 20 km downstream, which has only started to recover 17 years ago. In 1997 a 10-year flood (a flood that would be expected to occur about once in 10 years) again resulted in some sedimentation in the pools, reversing the trend of recovery for a few years. By 2009 the number of pools is again increasing in both reaches, but has not reached the pre-flood level seen in 1995. We are continuing channel surveys to understand processes of channel recovery.
Salmon depend on clean gravel and deep pools for spawning and rearing. The duration of sediment impacts in Redwood Creek has affected multiple life cycles of salmon and steelhead. The persistence of sediment impacts has implications for river restoration. Once sediment enters a river channel, it can take decades to be routed downstream to the ocean. In steep, confined mountain channels, in-channel restoration opportunities are limited by access. In the case of Redwood Creek, the NPS has focused restoration work on addressing hillslope erosion problems rather than attempting to modify the river bed. The goal of hillslope erosion-control work is to reduce sediment supply to Redwood Creek in future storms, but the effectiveness of this work in preventing road failures, landslides, and gullying has not yet been tested by a large (25-year) event.
To track the fate of large sediment loads in salmon-bearing rivers in northern California.
USGS Contact For This Project
Madej, M.A., and Ozaki, V., 2009, Persistence of effects of high sediment loading in a salmon-bearing river, northern California, in James, L.A., Rathburn, S.L., and Whittecar, G.R., eds., Management and Restoration of Fluvial Systems with Broad Historical Changes and Human Impacts: Geological Society of America Special Paper 451, p. 43–55, Boulder, CO. doi: 10.1130/2008.2451(03).
Madej, M. A., Bundros, G. and Klein, R. 2012. Assessing effects of changing land use on sediment loads in Panther Creek, north coastal California. pp. 101 – 109 in Proceedings of coast redwood forests in a changing California: A symposium for scientists and managers. Standiford, R. B.; Weller, T. J.; Piirto, D. D.; Stuart, J. D, technical coordinators. Gen. Tech. Rep. PSW-GTR-238. Albany, CA: Pacific Southwest Research Station, Forest Service, U.S. Department of Agriculture.
Madej, M. A. 2011. Temporal and Spatial Distribution of Landslides in the Redwood Creek Basin, Northern California p. 149-154 in Observing, studying, and managing for change--Proceedings of the Fourth Interagency Conference on Research in the Watersheds, U. S. Geological Survey Scientific Investigations Report 2011-5169, Medley, C.N., Patterson, G. and Parker, M. J. eds. 202 p.