Masters Thesis

A sediment budget for McDonald Creek Watershed, Northwestern, California

I constructed a sediment budget for McDonald Creek Watershed for the period of 1936 to 1997, to develop an understanding of how the watershed responds to various natural and anthropogenic influences. The results of the sediment budget are the following: 1) erosion rates increased from historical land-use practices, 2) the increase of sediment delivery to channels occurred during large storm events, resulting in channel aggradation, and 3) increased sediment production resulted in an increase in sediment yield and delta growth. The basin has experienced extensive timber harvest activities and road building. Eighty percent of the upper watershed had been logged by the late 1950's with a road density of 4 km/km2. The history of land management activities, in conjunction with four major storm events producing annual maximum peak discharges greater than a 10-year recurrence interval, resulted in a sediment production rate of 581,200 Mg over a 61-year period. The measured sediment inputs included mass wasting failures (non-road related), streambank erosion and soil creep, in-harvest unit surface erosion, and road-related erosion (including mass failures) accounting for 10, 23, 7, and 60%, respectively, of total sediment inputs. Quantitative and qualitative analyses indicated increasing sediment storage. Both the valley bottom main channel and North Fork McDonald Creek have been increasing in storage from 1941 to 1997. Storage has increased by 11,100 Mg within the sediment budget period. Air photo analysis of the channel over a 700 m reach demonstrated an approximate doubling in channel width in response to the 1964 storm. The channel width subsequently decreased, but remained widened. The increase in sediment storage may be indicative of the increase in sediment inputs in correlation with major storm events. The total sediment delivered from the hillslopes to the delta in McDonald Creek basin was 570,100 Mg during the period of 1936 to 1997, equating to 680 Mg/km2/yr. Analysis of delta front growth demonstrated an increase in delta area for each photo period, which temporally correlates with McDonald Creek sediment discharge estimates and storm history.

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