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<br />20 <br /> <br />was probably the ground surface forseveral years: Horizontal bedding <br />surfaces show that vertical accretion has occurred since germination. Tree <br />rings indicate thatgermination occurred in 1959 or 1960, at a level <br />approximately 0.7 m below the organic layers. Root crowns on the lower <br />organic layer date to approximately 1965. The sand above these organic layers <br />occurs in two thick layers, indicating it was deposited during two vertical <br />accretion events. Ripple drift cross-stratification is extremely variable in these <br />two layers, indicatil1g that bedform migration occurred in many directions during <br />deposition. <br /> <br />Effective Discharge <br /> <br /> <br />Effective discharge curves were computed for the time periods 1906-29, <br /> <br /> <br />1930-57, and 1962-present. Sediment transport relations were those computed <br /> <br /> <br />by Andrews (1986) for all sediment sizes, and the duration of mean daily <br /> <br /> <br />discharge was divided into 35 unequal increments. Andrews (1986) computed <br /> <br /> <br />a single effective discharge curve for the pre-dam period, but when calculated <br /> <br /> <br />separately, the curves before and after 1930 are very different (Fig. 15). The <br /> <br /> <br />modal value for the period 1910-1929 is 1077 m3/s and the value for the period <br /> <br /> <br />1930-1957 is 675 m3/s. This decrease of more than 50 percent is primarily <br /> <br /> <br />associated with natural climatic variability. Dam':'induced changes in flow <br /> <br /> <br />duration further reduced the effective discharge to 494 m3/s and caused a bi- <br /> <br /> <br />modal distribution with a secondary mode at 166 m3/s. <br />