Graduate Thesis Or Dissertation
 

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  • Time trends in flow and channel characteristics were evaluated for the Middle Fork Willamette (MFW) River, which drains a 668 km2 forested watershed in the Cascade Mountains of western Oregon. Timber production is the primary land use in the watershed. Analysis of precipitation and peak flow data from 1959 to 1980 suggests that peak flows (greater than 0.15 m3skm2) increased 2% per year as timber harvesting and road building expanded in the basin. Landslides associated with roads and clearcuts, based on an inventory of aerial photographs dating from 1959 to 1972, were 27 and 23 times more frequent (respectively) than in forested situations. The majority of landslides appear to have been initiated during the relatively large flood of December 22, 1964. Changes in channel planform morphology from 1936 to 1980 were documented from aerial photographs. Based on the photographic record, channel width increased prior to 1967 followed by a decreasing trend from 1967 to 1980. Sixty-five channel cross-sectional profiles were surveyed during summer low flows in 1979 and 1980 to provide detailed measurements of existing channel conditions. The presence or absence of aggradational features was noted at each cross section. Regression analysis revealed that significantly greater (c= 0.05) channel widths were recorded for 62% of the aggraded reaches compared to the dimensions of nonaggraded reaches. The results of the cross-section surveys, combined with the channel morphology and landslide measurements, suggest three conclusions: (1) major aggradation along the NW River occurred during the 1964 flood, (2) landslides associated with the 1964 flood contributed to the development of aggraded locations along the river, and (3) the majority of landslides during this period occurred within one geologic zone and were associated with land use activities, thus suggesting management activities, particularly roads, influenced changes in channel morphology.
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