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<br />I <br />I.. <br />I, <br />I <br />I <br />I <br />I <br />I <br />1-. <br />f, <br />I <br />I <br />I <br />I <br />I <br />I <br />I' <br />I <br />I <br /> <br />The river originates along the Continental Divide i'n the San <br />Juan Mountain range at elevations generally above 10,000 feet. <br />Along the Continental Divide, the river is fed by perpetual <br />snow fields and springs. From its source unt il it reaches the <br />Alamosa stream gage, the river drains approximately 1,710 <br />square miles. The Rio Grande meanders through the San Luis <br />Valley, a gently sloped plain surrounded by high mountain <br />ranges. The San Luis Valley is about 100 miles long and about <br />60 miles wide. The valley is surrounded by the Sangre de <br />Cristo Mountains on the east, the San Juan Mountains on the <br />west, and the La Garita Mountains on the north. The Rio Grande <br />is a perennial river. <br /> <br />3.0 FLOOD HISTORY <br /> <br />Flood ing on the Rio Grande can result from snowmel t runoff, <br />general rains, cloudburst storms, or a combination of those <br />condi tions. The runoff from snowmelt occurs during the period <br />from late May to early July. This is the time of the year that <br />the river is most._susceptible to flooding, especially if there <br />are warmer than usual temperatures and the snowmelt is <br />characterized by moderate peak flows, long duration, and large <br />volumes of water. Most of the annual rainfall ,occurs during <br />the months of July through October. The intensity of rainfall <br />is moderate to high', and the runoff is characterized by high <br />peak flows of short duration with relatively small volume~ of <br />water. FOllowing are notable floods of record at various <br />locations (Table 1). <br /> <br />Table 1. Notable Floods on the Rio Grande <br /> <br />Date <br /> <br />Location <br /> <br />DiSCharge <br />CUbic Feet/Second <br /> <br />1869 <br />Spring 1884 <br />June 5, 1895 <br />Oct. 5, 1911 <br />June 13, 1921 <br />June 29, 1927 <br />May 22, 1948 <br />June 19, 1949 <br />May 30, 1979 <br />June 9, 1985 <br />June 7, 1986 <br />May 16, 1987 <br /> <br />Alamosa <br />Alamosa <br />Del Norte <br />Del Norte <br />Del Norte <br />Del Norte <br />Monte Vista <br />Monte Vista <br />Del Norte <br />Del Norte <br />Alamosa <br />Alamosa <br /> <br />~ <br /> <br />Unknown <br />20,000 <br />10,000 <br />18,000 <br />9,630 <br />15,000 <br />7,100 <br />6,650 <br />8,060 <br />9,040 <br />4,580 <br />5,200 <br /> <br />Of the several recorded floods, all but one occurred during the <br />months of May to July. The flood of June 1884, caused <br />primarily by extraordinary snow cover, lasted much longer than <br />the others. The river was at overflow stage at Del Norte from <br /> <br />-2- <br />