Graduate Thesis Or Dissertation
 

Patterns of hillslope and channel recovery following disturbances in steep, forested basins

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  • 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.
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