Many rivers have been disturbed by increases in sediment loads from activities such as mining, grazing, agriculture, logging, and road construction. Redwood Creek, Humboldt County, north coastal California, is currently listed as sediment impaired under the Clean Water Act because of past timber harvest, widespread landsliding, channel sedimentation, and burial of streamside redwood trees. USGS scientist Dr. Mary Ann Madej with National Park Service colleague Vicki Ozaki report results from a 30-year channel monitoring effort to document recovery in Redwood Creek in a newly published book, Management and Restoration of Fluvial Systems with Broad Historical Changes and Human Impacts. The USGS established a channel monitoring program in the mid-1970s to track the fate of fresh flood deposits. The USGS and NPS continued to monitor channel changes over the next three decades through surveys of channel width, bed elevation, pool depths and frequencies, and analyses of the gravel particle size on the river bed.

The history of the flood sediments differed between upstream and downstream reaches of the river. Channel filling was especially severe in the headwaters, but by the mid-1980s the channel 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 lower Redwood Creek, channel filling continued through the 1980s 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. As the channel has incised, pool frequencies and depths have increased, but a 10-year flood in 1997 again resulted in some sedimentation, reversing the trend of recovery for a few years.

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 through the system to the ocean. In steep, confined mountain channels, in-channel restoration opportunities are limited by access. In the case of Redwood Creek, restoration work has focused 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.

Redwood Creek, California, looking upstream at eroding fine-grained flood deposits dating from 1964, with coarsened bar in foreground. For scale, survey rod is 7.6 m (25 ft) tall. Photo: V. Ozaki, NPS.

Management Implications

• Land managers in downstream portions of a watershed may have to deal with sediment impacts decades after sediment entered the river in upstream areas.
• Long-term (> 30 years) monitoring has value in revealing a story of river recovery that would not have been possible with a short-term effort.
• The effectiveness of recent hillslope erosion-control work in protecting rivers from sedimentation has not yet been tested by a large storm.

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).

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