PERSISTENCE OF EFFECTS OF HIGH SEDIMENT LOADING IN A SALMON-BEARING RIVER, NORTHERN CALIFORNIA

In order to effectively design river restoration projects, it is important to understand the persistence of the problem that requires mitigation. Unfortunately in many cases the expected duration of anthropogenic effects is unknown. Thirty years of detailed channel monitoring in the Redwood Creek watershed, north coastal California, sheds light on this dilemma. In the 1960's and 1970's, during 25- to 50-year return period floods, extensive erosion of logged hillslopes resulted in channel aggradation several meters deep along much of Redwood Creek. Since that time, in upstream reaches, vertical incision and lateral erosion of these deposits have begun to unearth the previous channel bed and banks, and pools are deepening. In the downstream reaches, monitoring indicates channel recovery is much slower and more restricted. In 1997, a 10-year flood resulted in the reversal of the recovery trend by initiating about 300 landslides and renewing channel aggradation. By 2007, the channel had returned to conditions similar to those before the 1997 storm. Nevertheless, full recovery in terms of channel bed elevation and width has still not occurred. Due to the persistence of the impacts, Redwood Creek is currently listed as sediment- and temperature-impaired under the Clean Water Act, and three of the four native salmonid species in Redwood Creek are listed as threatened. The persistence of elevated sediment loads in this river has impacted multiple life cycles of salmon. Because 32 years have passed since the last 25-year flood, it is likely that the channel will be impacted by another large flood before it fully evacuates all the recent flood deposits. In the case of Redwood Creek, reducing hillslope erosion rates through land management activities is probably more effective in diminishing future sediment impacts on critical salmon habitat than in-channel restoration work following large floods.