Together, increased soil water content and higher peak flows may help explain increases in bedload export from CC-3 after logging (Swanson and Swanston 1977).

1.

2.

3.

4.

5.

Summary

By reducing evapotranspiration and increasing overland flow, three
types of timber harvest increased annual water yield up to 36 cm.
For 5 years after logging, yield increases averaged 29 cm (43 percent)
at CC-3, the completely clearcut watershed; 9 cm (14 percent) at
CC-2, where timber was harvested in 20 small clearcuts; and 6 cm (8
percent) at CC-1, where timber was shelterwood cut.

Largest absolute increases in yield occurred during the winter at
all watersheds. Largest relative increases occurred in fall and

summer.

Sizable regional increases in water yield do not appear to be a product of sustained yield forest management practiced by the U.S. Forest Service in southwestern Oregon. Yield increases are as permanent as the changes in the forest hydrologic system that cause them. Large yield increases in recently logged headwater basins appear to be overshadowed by normal water yield from uncut or reforested basins so that yield increases for a large parent watershed probably amount to only about 4-6 percent.

After logging, size of peak flow corresponding to a peak flow of
10.9 liters/sec.ha at CC-4 was increased about 48 percent at CC-1,
35 percent at CC-3, and 11 percent at CC-2.

Increases in size of peak flow appear to be related to amount of
watershed area where soils were severely disturbed. Soil disturbance
and interception of subsurface water by roadcuts and ditches may
affect surface erosion, stream channel erosion in headwater areas,
and peak flows used to select culvert sites.

Acknowledgments

We thank W. Scott Overton of Oregon State University for his statistical advice and Robert L. Beschta of Oregon State University, Richard J. Janda of the U.S. Geological Survey, and Gerald W. Swank and other National Forest hydrologists for reviewing this manuscript. We are indebted to Alfred B. Levno and Phyllis M. Frazer for streamflow data collection and reduction.

Bethlahmy, Nedavia.

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22

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