|Abstract or Summary
- Geomorphic response to watershed disturbances commonly results in
alterations of landforms. Subsequent geomorphic recovery is dependent on the ability
of flows to entrain, transport, and organize inorganic and organic material on hillslopes
and in channels. This research analyzed changes in sediment production, channel
structure, and channel organization following disturbances in steep, forested terrain.
Examining a range of channel environments following sediment pulses permitted the
development of a conceptual model predicting trajectories of channel change.
Extensive road construction and timber harvest on steep, forested hillslopes in
the Redwood Creek basin, north coastal California, caused increased sediment yields to
streams. This study examined the effectiveness of erosion control efforts on these
disturbed hillslopes following a 12-year storm. The erosional response of treated logging
roads was strongly related to their geomorphic setting: roads on gentle, convex, upper
hillslopes contributed much less sediment than roads on steep, lower hillslopes (10 and
550 m³ of sediment/kilometer of treated road, respectively). Since 1980, 300 km of
treated roads contributed about two percent of the total sediment load of Redwood
Creek, north coastal California. Minimal treatment of roads in upper hillslope positions
was effective in reducing sediment production, but more intensive treatments are needed
for roads in middle and lower slope positions.
The response of channels to sediment pulses was also examined. Channels
exhibited self-organizing behavior as they processed previously random hillslope inputs
into regularly spaced bedforms at characteristic spatial scales. Variability and spatial
patterns of channel bed elevations were related to the dominant bed material, presence of
wood, channel gradient, and time since disturbance. Typical trajectories of change were
increased magnitude and variability of water depths, and increased number, size, and
regularity of bedforms. Concurrent with increasing development of bedforms was an
increase in channel roughness.
The time scale of recovery varied with channel type. The frequency of flows
capable of reorganizing bed material in steep, coarse-grained channels was low, and
consequently the development of channel structure and organization was slow. In
contrast, lower gradient rivers with easily mobilized gravel beds attained regular bar and
pool spacing within two decades of a sediment pulse.