Effects of Clearcut Logging on Channel Form in a Coastal Redwood Forest

The morphological characteristics of channels were mapped in recently logged areas and in a mature second-growth redwood forest at Caspar Creek, California, USA, to evaluate the extent to which logging has influenced channel conditions. Channels are pervasively incised in the area. Gullying appears to have originated with first-cycle logging of the late 1800s, and sequences of headcuts have continued to migrate up channels since then. Second-cycle logging of the early 1990s in the North Fork Caspar Creek is associated with increased spacing between headcuts in channels within clearcut tributary watersheds, suggesting that some discontinuous incisions have coalesced. Upstream channel limits in logged watersheds are now located significantly farther upslope than in control watersheds, and the magnitude of observed increases in peakflows and storm runoff is consistent with the observed change in drainage density. Relations between channel morphological variables and indices of stream power show higher variance in logged watersheds than in controls, suggesting that previously established channel forms have been disrupted after logging. Correlations between suspended sediment yield and indices of gully and surface erosion suggest that in- channel erosion associated with hydrologic change is the dominant source of post-logging sediment during years lacking major landslides. Common sediment-control measures, such as use of riparian buffer strips and control of road surface erosion, would not be effective for reducing sediment input from this in-channel source.