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,j 17 thunderstorms which can raise sediment loads of the river, while winter precipitation is generally in the form of light rain or snow. Mean monthly maximum and minimum temperatures range from 20C (360F) and -lDoC (140F) in January to 330C (920F) and 170C (630F) in August (Figure 2.3) \ (World Weather Disc, 1994). Vegetation consists of desert shrubs such as shadscale (Atriplex confertifoli), saltbush (Atriplex sp.), and galleta grass (Hilaria jamesii) (Weber, 1976; Schumm, 1964). The riparian community is primarily composed of the nonnative tamarisk (Tamarisk chinensis) and russian olive (Elaeagnus angustifolia), and the native sandbar willow (Salix exigua) and cottonwood (Populus deltoides). Land use in the valley is primarily agricultura1. Due to the arid climate, most of the agriculture is supported with water from the Colorado River. A substantial amount of water returns to the river carrying dissolved salts, selenium, and pesticide residues which pose a potential hazard to aquatic biota (Butler et a1., 1991). In addition to agriculture, several industrial and urban developments occur along the river. These developments undoubtedly affect water quality to some degree, but their greatest impact on the river system is from dikes and bank stabilizing structures which are built to prevent flooding. The study area is separated into two distinct reaches: 1) the 15 mile reach upstream of the Gunnison River; and 2) the 18 mile reach below the confluence (Figure 2.1). The reaches extend from river mile 185 (measured from the confluence of the Green and Colorado rivers) at the Grand Valley diversion structure to river mile 153 above Horsethief >.~ . . uJ I ,;,..J .~: '.:...: ,.\ ~I' 'Of.. 'j'.- ,......... =4~ '1 :"1 .J OJ ;,] -J ill J...... .. U "j .~. J J I J canyon.