LESSONS LEARNED FROM 60 YEARS OF GEOMORPHIC RESEARCH IN A DISTURBED FORESTED WATERSHED (Invited Presentation)

The story of Redwood Creek, north coastal California, is one of landscape disturbance, recovery and resilience, uniquely documented by 60 years of measurements of geomorphic changes. Prior to 1945, 85 percent of the 720 km² watershed was blanketed with virgin redwood (Sequoia sempervirens) and Douglas fir (Pseudotsuga menziessi) forests, but by 1997 80 percent of these forests had been logged. Large floods in the 1960s and 1970s initiated thousands of landslides and road failures on logged hillslopes, and the resulting sediment input caused the river channel to aggrade up to 8 m in many reaches. Since 1975, Redwood Creek has downcut through these deposits of sand and gravel. Rates and spatial distribution of channel recovery vary across the watershed, partly as a function of hillslope-channel connectivity. Although the most rapid period of channel recovery occurred within the first decade of disturbance, the impact of excess sediment on channel conditions has persisted for more than 50 years. More recent storms resulted in additional slope failures, but not nearly the magnitude of the previous period.

Basin-wide assessments of erosion and sediment sources led to the design and implementation of one of the largest watershed restoration programs in the US. The focus of the program is on reducing hillslope erosion rates and reestablishing a healthy forest ecosystem. To date 560 km of abandoned logging roads have been removed, concurrent with thousands of hectares of second-growth forest being restored through thinning overly dense stands of trees and prescribed burns.

Studies of the types and magnitudes of channel responses throughout the river network over decades have been used to assess aquatic habitat, wood loading, streamflow regimes, the connectivity of hyporheic and surface flow, water quality, sediment loads and riparian conditions. These studies help connect diverse research communities to assess on-going concerns such as: the effectiveness of watershed restoration treatments and revised forest management policies, carbon sequestration, ecohydrologic responses to climate change, and risks from landslides, flooding, drought and wildfire. This watershed-scale project highlights the importance and relevance of long-term studies integrating geomorphology, hydrology and forest and aquatic ecology.