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Annual and Monthly Flow Characteristics Annual flow varies substantially at all three stations (fig. 10). Variability is low at Colorado River below Baker Gulch (coefficient of variation of 0.33) because Streamflow at the station is derived from snow, which accumulates over a season, and thereby is less variable. Variability is highest at West Salt Creek near Mack (coefficient of variation of 1.03) because stream- flow primarily is derived from highly variable thunder- storms. Variability also is low at the Colorado River near Cameo (coefficient of variation of 0.30) because most Streamflow is derived from snow, and annual flows have been affected by reservoir regulation and interbasin water transfers. Monthly flow also varies substantially at the three stations (fig. 11). The Colorado River below Baker Gulch has snowmelt runoff from April through July, which is superimposed on a perennial base flow. The peak runoffperiod is May through June. West Salt Creek is an ephemeral stream, and Streamflow results primarily from rainfall runoff. About 45 percent of the mean annual Streamflow at West Salt Creek occurs during August, September, and October, whereas at stations where snowmelt predominates, the percentage of the mean annual streamflow during those 3 months is about 12 to 18 percent. The Colorado River near Cameo has increased runoff during the months of April through July because of snowmelt, but base flow is maintained at a larger, more constant level because of reservoir releases and irrigation-return flows. Floods and Droughts Streamflow, including flood flows, generally is more variable on low-altitude streams than on high- altitude and mixed-type streams in the study unit. High flows on high-altitude streams in the mountains tend to be less variable and of longer duration. They primarily result from snowmelt during late spring and early sum- mer. Although the magnitude of these floods can be quite large, exceptionally large snowmelt floods that could cause severe flooding are very uncommon. Because of the annual nature of snowmelt floods, most stream channels are capable of carrying these snow- meltfloods without extensive bank overflow or sub- stantialflooding (Chaney and others, 1987). Reservoir storage, interbasin water transfers, and local diversions for irrigation also diminish the magnitude of the annual snowmelt floods. Frequency curves of annual maximum mean daily Streamflow (fig. 12) indicate the probability that a given maximum mean daily Streamflow will be equaled or exceeded in any given year. For example, there is a 10 percent probability that mean daily Streamflow of 30,000 ft3/s would be equaled or exceeded in any given year at the Colorado River near Cameo. Differences in the vertical position of these curves indicate differ- ences in Streamflow for an exceedance probability and primarily are the result of differences in drainage-area size (table h). The steepness of the curve for West Salt Creek relative to the curve for Colorado River below Baker Gulch indicates that maximum mean daily streamflows are more variable on low-altitude streams than on high-altitude and mixed-type streams. The annual maximum mean daily Streamflow curve for West Salt Creek is approximate because it is based on only 10 years of record. This short period of record affects the accuracy with which floods having a large probability of exceedance can be predicted. The slope of the curve for this station could change considerably with additional years of Streamflow record. Frequency curves of annual minimum mean 7-day Streamflow (fig. 12) indicate the probability of nonexceedance between flows smaller than a specified magnitude. Low-altitude streams have extended peri- ods of noflow and cannot be meaningfully analyzed for probability of nonexceedance. Therefore, West Salt Creek, which had at least 288 consecutive days of no flow recorded, is not shown. Low flows in high- altitude and mixed-type streams are sustained prima- rily by ground-water discharge, but gradual melting of perennial snowfields also provides some base flow. The shape of the annual minimum mean 7-day Stream- flow curves for the Colorado River below Baker Gulch and Colorado River near Cameo are similar. The greater magnitudes of low flow at Cameo for a specific probability of nonexceedance are a result of a larger drainage area, tributary inflows, and water-develop- ment factors such as reservoir releases, interbasin water transfers, and in igation-return flows. At Cameo, 7 consecutive days of flows less than 1,050 ft'/s can be expected 10 percent of the time. Knowledge about expected frequency of certain low flows is important because of the detrimental effects on stream biota resulting from dissolved-oxygen depletion and increased concentrations of dissolved constituents. Human Effects On Streamflow The natural hydrology of the Upper Colorado River Basin has been considerably altered by water development, which includes numerous reservoirs and diversions. The quantity of water removed from the basin by large interbasin water transfers to the South Platte, Rio Grande, and Arkansas River Basins was about 585,000 acre-ft in water year 1993. 20 Environmental Setting and Implications on Water Quality, Upper Colorado River Basin, Colorado and Utah