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<br />daytime, a~e usually warmer than those found at sillil.ar or lowe~ <br />elevations. Periods of extreme ~old temperatuxes are of short <br />duration. The mean annual temperature is 43.l.oF. The average <br />precipitation varies from 9.41 inches at Del Norte to over 40 <br />inches in the higher mountain elevations, where most of the preci- <br />pitation occurs as snow. <br /> <br />Vegetation varies considerably according to the elevation in the <br />basin. Vegetation below 8,000 feet consists of pinion pine, juni- <br />per, and sage with sparse grass ground cover. Increased precipi- <br />tation supports dense pine and aspen forests between this elevation <br />and the timberline. Rock outcrops and tundra grasses predominate <br />on the high mountain peaks, where climatic conditions are too seve~e <br />for forest growth (References l, 2, and Jl. <br /> <br />2.3 Principal Flood Problems <br /> <br />Flooding in the study area can result from snowmelt runoff, general <br />rainfall, cloudburst storms, or any combination of the three. <br /> <br />Snowmelt runoff occ"rs from late May to ea~ly JUly. During this <br />time, the' study area is most susceptible to fl.ooding, especially <br />when tempe~atures are highe~ than normal or snowmelt is augmented <br />with rain. This type of flooding has a long-duration r"noff that <br />results in a continuous rise in water level, producing high volumes <br />of r"noff. <br /> <br />Most of the annual rainfall occurs from July to October. Resultant <br />flooding is of short duration, characterized by high peakS and low <br />volumes of runoff. <br /> <br />The potential for the most severe flooding is created by the combi- <br />nation of rapid snOWD.elt with long-duration rainfall. D.lration of <br />flooding will. vary, depending on tile season in which flooding ooc"rs. <br />During the spr in~, when the streams are high because of snoorntelt <br />r"noff coupl.ed with sp~ing rains, l1igh water coul.d persist for <br />several days. During late SUllllner, when the streams are normally <br />low, runoff from thunderstorms will recede in a few hours. <br /> <br />Floodwaters flowing at high velocities create dangerous conditions. <br />Velocities exceeding 3 feet per second with depths of 2 feet or <br />more are generally considered hazardous. Those conditions would <br />exist at several locations in the study area, especially in the <br />river channels and low areas within the flood plain. <br /> <br />Detailed information on floods before 1900 is very limited. Infor- <br />mation on past flooding is based on newspaper acco"nts and inter- <br />views with longtime residents of the area. Large floods occurred <br />in Rio Grande County in 1884, 1905, 1911, 1927, 1949, 1957, 1970, <br />and 1973. These floods caused major damage; disrupted highway and <br />railroad traffic and communication services; drowned livestock; <br /> <br />5 <br />