Abstract:
Sediment yield increases following timber harvest and road construction
were predicted for four Pacific Northwest experimental
watersheds. Sediment yields were predicted using a handbook developed
by the U.S. Forest Service entitled, "An Approach to Water Resources
Evaluation of Non-Point Silvicultural Sources" (WRENS). Predicted
sediment yield increases varied from 40 to 180 percent of measured
increases.
Several conceptual problems and difficulties in the application
of the procedure responsible for observed differences between predicted
and measured sediment yield increases were identified. Major
problems included (1) sediment budgeting inaccuracies, and (2) no consideration
of delays in the export of predicted sediment inputs to
stream channels due to in-channel sediment storage.
The accuracy and applicability of the WRENS procedure for estimating
actual sediment yield changes in forest management planning is
limited by (1) the unknown arid complex nature of several erosion and
sediment transport processes, and (2) temporal variability and the
probabilistic nature of rainstorms of extreme magnitude and their
effect on erosion and sediment transport processes.
In an effort to improve prediction accuracy and applicability of
the procedural handbook, several recommendations are made including:
(1) discontinue use of the channel sediment source prediction procedure
in rain-dominated areas of the Pacific Northwest, and (2) areawide
inventories to assess the effects of silvicultural activities on
landslide rates.
