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-. I ! The hund!reds of plateaus in the region are more or less sharply separate~. Some rise, table-like, above their neighbors on all sides; o~hers adjoin their neighbors as steps or terraces; and some are basin-like, with surrounding infacing cliffs. They also differ in their degree of dissection by streams; as a result of differing elevations and stages in the erosion cycle, the various plateau formations may differ greatly in temperature, rainfall, and vegeta- tion. All stre~s in Mesa County are either direct or indirect tributaries I to the Colorado River, which traverses the north-central and north- western sectors. From the northern county line, the river flows southwes~erly for 41 miles to the mouth of the Gunnison River, thence northwesterly 27 miles, and again southwesterly for 15 miles in its r~maining course in the county. In general, the systems of the Dolo~es and Gunnison Rivers and West Creek drain the western, southwes~ern, and south-central portions of the county. The Plateau Creek sYFtem drains the eastern sector, except for the easternmost portion, I which is drained by the Divide Creek system, a northerly flowing btream that joins the Colorado River in Garfield County. A group of I minor creeks and washes flowing southerly from the F~an and Bookl Cliffs regions drain the northwestern portion of the, county, ~nd a group of similar streamways convey drainage to the , river from the north-central portion. Information on drainage areas and stream gradients of the streams under study is shown in , Table 2.1 The climkte in Mesa County is directly influenced by the varying topographic features. The lower valley areas of the northwest are arid to ~emiarid, whereas the high, mountainous terrain of the northeast is subject to moderately heavy precipitation. Due to storm sy~tems moving from west to east through the region, the temperat~re varies greatly from summer to winter, and rapid changes of weather occur. Climatic variations may prevail within compara- tively s*all areas due to greatly differing topography. Elevation directly I influences precipitation, with the Grand Valley area and the Gunn~son River valley receiving less than 10 inches annually, the high~r mesas receiving from 10 to 20 inches, and the mountains of the nbrthwest receiving more than 40 inches. Occurrence of precipit~tion is extremely variable, with a large part of the total I concentr~ted in several months. Late summer convective-type cloud- burst st0rms of small areal extent and early fal~general rain over large ar~as normally make August, September, and October the wettest months 0+ the year. Most wintertime precipitation occurs as snow and, in the higher elevations, a deep snowpack normally accumulates. Average ~nowfall ranges from approximately 19 inches at Fruita and Palisade:to approximately 72 inches at Collbran. Snowfall is generally dominated by a few large storms. Snowpack ordinarily begins i~ la.te October and snowmelt in late April; snowmelt con- tinues t*rough early July. -= 7 ~._--- .~~~- ... ~- .,"",,-.._-~~.._-~ j ~..-