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<br />33 <br /> <br />figure 2. The northern portion of the watershed has steep slopes and unstable <br />rock structures with high landslide susceptibility. The southern portion has <br />relatively low erosion potential, due to dense vegetation cover and stable <br />granitic soils. The southeastern part is covered by loose, sandy soils with <br />high erosion potential. <br /> <br />2. San Diequito River. The San Dieguito River flows through San Diego County <br />in southern California, and flows through the City of Del Mar into the Pacific <br />Ocean. The approximately 2-mi long study reach, del ineated in figure 3, was <br />innundated by recent floods, including those of March 1978 and February <br />1980. The reach shown in the figure is approximately 4 mi from the Pacific <br />Ocean and 5 mi below Lake Hodges Dam, which was constructed in 1918. The <br />drainage area above Lake Hodges is about 300 sq mi. Ouring the 15 March 1978 <br />flood, a peak flow of 4,400 cfs was recorded downstream from the reservoir. <br />An estimated peak reservoir outflow of 22,000 cfs, corresponding to a 40-yr <br />flood, was recorded during the 21 February 1980 flood. The SDR channel has a <br />wide, flat cross section with highly erodible banks, as can be seen in figure <br />4, an aerial photograph taken above the Via de Santa Fe Road Bridge durin9 the <br />21 February 1980 flood. This photograph was suppl ied by San Diego County <br />Flood Control District through Dr. Howard Chang of SDSU. The river channel <br />had been disturbed prior to the 1978 and 1980 floods by sand-mining activities <br />and construction of the Via de Santa Fe Road and its SDR bridge. Several <br />large borrow pits, with depths up to 25 ft, were produced by sand-mining <br />operations. Although these borrows were partially refilled after the 1978 <br />flood, major borrow-pit aggradation took place during the 1980 flood. The <br />channel bed is composed of primarily sand-range materials. <br /> <br />3. Salt River. The Salt River is located in Maracopa County, Arizona, and <br />flows from Granite Reef Dam to the confluence with the Gila River. A reach of <br />the river through the City of Phoenix has drawn the most attention because <br />recent development within the flood plain has resulted in recurrent damage to <br />structures and facilities. SR experienced four major floods in three years <br />between 1978 and 1980 (March 1978, peak flow = 99,000 cfs; December 1978, peak <br />flow = 112,000 cfs; January 1979, peak flow = 73,500 cfs; and February 1980, <br />peak flow = 185,000 cfs) which produced extensive damage to the Sky Harbor <br />