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<br />Vegetation in the study area varies according to elevation. In <br />the valley areas below 8,000 feet not used for agriculture, <br />vegetation is classified as northern desert shrub; that is, <br />sagebrush, western wheatgrass, and winterfat, with an understory <br />of various hardy grasses. Due t-"-thepcor ground-cover in this <br />region, snow tends to melt and run off rapidly. <br /> <br />At the higher elevations, from approximately 8,000 feet to <br />timberline, sub-alpine forests consisting of aspen, lodgepole <br />pine, Douglas fir, and Engleman spruce are coounon. Runoff in <br />this region is quite variable due to the mixture of rock outcrops <br />and dense pine needle beds (Reference 3). <br /> <br />, <br /> <br />The climate of Delta County is classified as generally semi-arid, <br />with an abundance of sunshine, frequent wind, and low humidity. <br />The prevailing direction of air movement to the region is from <br />the west. On arrival, airmasses from the west are comparatively <br />dry due to long travel over land and loss of moisture over <br />mountain ranges. However, the primary sources of moisture are <br />the Pacific Ocean and the Gulf of Mexico. Generally, airmasses <br />from the Pacific Ocean dominate from October through April, while <br />airmasses from the Gulf of Mexico dominate during late spring and <br />summer. <br /> <br />~ <br /> <br />Historical climatologic information for the entire Delta County <br />study area is very sparse. However, the mean temperatures in the <br />City of Delta area range from approximately 26.40F in January to <br />approximately 74.loF in July. Cooler temperatures prevail in <br />higher mountain areas. The growing season usually extends upward <br />to 140 days (Reference 4). <br /> <br />Normal annual precipitation varies with elevation and ranges from <br />approximately 7.9 inches at the City of Delta to an estimated <br />40 1nches at higher elevations in the study basins (Reference 4). <br />Most of the annual precipitation in high elevation areas occurs <br />as snow from December to April and, beginning in October, a deep <br />snowpack accumulates. Snowmelt begins in late April and <br />~c_oIlt:i_nlles well illt ~_u,JIllne u()r earl L .[u!Y,-Gener al_"..i!,_ may~occur <br />over the study area, and convective-type cloudburst storms <br />frequently occur in summer. <br /> <br />The floodplains of all the streams studied are <br />undeveloped, except for residential, coounercial, <br />industrial development within the communities along <br />reaches. <br /> <br />primarily <br />and light <br />the study <br /> <br />2.3 Principal Flood Problems <br /> <br />Major flooding in the Delta County area has been the result <br />of rapidly melting snow, sometimes augmented by general rain, <br />during the period from May through July. Snowmelt flooding is <br />characterized by moderate peak flows, large volume of runoff, <br />long duration, and marked diurnal fluctuation of flow. Flooding <br />from general rain is characterized by high peak flows and <br /> <br />6 <br />