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daytime, a~e usually warmer than those found at sillilar or lowe~ elevations. Periods of extreme ~old temperatuxes are of short duration. The mean annual temperature is 43.loF. The average precipitation varies from 9.41 inches at Del Norte to over 40 inches in the higher mountain elevations, where most of the preci- pitation occurs as snow. Vegetation varies considerably according to the elevation in the basin. Vegetation below 8,000 feet consists of pinion pine, juni- per, and sage with sparse grass ground cover. Increased precipi- tation supports dense pine and aspen forests between this elevation and the timberline. Rock outcrops and tundra grasses predominate on the high mountain peaks, where climatic conditions are too seve~e for fo~est growth (References 1, 2, and Jl. 2.3 Principal Flood Problems Flooding in the study area can result from snowmelt runoff, general rainfall, cloudburst storms, or any combination of the three. Snowmelt runoff OCC<1rs from late May to ea~ly JUly. During this time, the. study area is most susceptible to flooding, especially when tempe~atures are highe~ than normal or snowmelt is augmented with rain. This type of flooding has a long-duration runoff that results in a continuous rise in water level, producing high volumes of runoff. Most of the annual rainfall oocurs from July to October. Resultant flooding is of short duration, characterized by high peakS and low volumes of runoff. The potential for the most severe flooding is created by the combi- nation of rapid snOWD.elt with long-duration rainfall. D.lration of flooding will vary, depending on tile season in which flooding OOC<1rs. During the spr in~, when the streams are high because of snoorntelt runoff coupled with sp~ing rains, lIigh water could persist for several days. Dur ing late SUlmner, when the streams are normally low, runoff from thunderstorms will recede in a few hours. Floodwaters flowing at high velocities create dangerous conditions. Velocities exceeding 3 feet per second with depths of 2 feet or more are generally considered lIazardous. Those conditions would exist at several locations in the study area, especially in the river channels and low areas within the flood plain. Detailed information on floods before 1900 is very limited. Infor- mation on past flooding is based on newspaper accounts and inter- views with longtime residents of the area. Large floods occurred in Rio Grande County in 1884, 1905, 1911, 1927, 1949, 1957, 1970, and 1973. These floods caused major damage; disrupted highway and railroad traffic and communication services; drowned livestock; 5