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<br />Annual and Monthly Flow Characteristics
<br />
<br />Annual flow varies substantially at all three
<br />stations (fig, 10), Variability is low at Colorado River
<br />below Baker Gulch (coefficient of variation of 0.33)
<br />because streamflow at the station is derived from snow,
<br />which accwnulates over a season, and thereby is less
<br />variable, Variability is highest at West Salt Creek near
<br />Mack (coefficient of variation of \.03) because stream-
<br />flow primarily is derived from highly variable thunder-
<br />storms, Variability also is low at the Colorado River
<br />near Cameo (coefficient of variation of 0,30) because
<br />most streamflow is derived from snow, and annual
<br />flows have been affected by reservoir regulation and
<br />interbasin water transfers,
<br />Monthly flow also varies substantially at the
<br />three stations (fig, II), The Colorado River below
<br />Baker Gulch has snowmelt runoff from April through
<br />July, which is superimposed on a perennial base flow,
<br />The peak runoff period is May through June, West Salt
<br />Creek is an ephemeral stream, and streanlflow results
<br />primarily from rainfall runoff, About 45 percent of the
<br />mean annual streamflow at West Salt Creek occurs
<br />during August, September, and October, whereas at
<br />stations where snowmelt predominates, the percentage
<br />of the mean annual streamflow during those
<br />3 months is about 12 to 18 percent. The Colorado
<br />River near Cameo has increased runoff during the
<br />months of April through July because ofsnowmelt, but
<br />base flow is maintained at a larger, more constant level
<br />because of reservoir releases and irrigation-return
<br />flows,
<br />
<br />Floods and Droughts
<br />
<br />Streamflow, including flood flows, generally is
<br />more variable on low-altitude streams than on high-
<br />altitude and mixed-type streams in the study unit, High
<br />flows on high-altitude streams in the mountains tend to
<br />be less variable and oflonger duration, They primarily
<br />result from snowmelt during late spring and early sum-
<br />mer, Although the magnitude of these floods can be
<br />quite large, exceptionally large snowmelt flood~ that
<br />could cause severe flooding are very uncommon,
<br />Because of the annual nature of snowmelt floods, most
<br />stream channels are capable of canying these snow-
<br />melt floods without extensive bank overflow or sub-
<br />stantial flooding (Chaney and others, 1987), Reservoir
<br />storage, interbasin water transfers, and local diversions
<br />for irrigation also diminish the magnitude of the annual
<br />snowmelt floods,
<br />Frequency curves of annual maximmn mean
<br />daily streamftow (fig, 12) indicate the probability thata
<br />given maximum mean daily streamflow will be equaled
<br />
<br />or exceeded in any given year, For example, there is a
<br />10 percent probability that mean daily streamflow of
<br />30,000 ft'/s would be equaled or exceeded in 'any given
<br />year at the Colorado River near Cameo, Differences in
<br />the vertical position of these curves indicate differ-
<br />ences in streamflow for an exceedance probability and
<br />primarily are the result of differences in drainage-area
<br />size (table 6), The steepness of the curve for West Salt
<br />Creek relative to the curve for Colorado River below
<br />Baker Gulch indicates that maximum mean daily
<br />streamflows are more variable on low-altitude streams
<br />than on high-altitude and mixed-type streams, The
<br />annual maximum mean daily streamflow curve for
<br />West Salt Creek is approximate because it is ba~ed on
<br />only 10 years of record, This short period of record
<br />affects the accuracy with which floods having a large
<br />probability of exceedance can be predicted, The slope
<br />of the curve for this station could change considerably
<br />with additional years of streamflow record,
<br />Frequency curves of annual minimwn mean
<br />7-day streamflow (fig, 12) indicate the probability of
<br />nonexccedance between flows smaller than a specified
<br />magnitude, Low-altitude stream~ have extended peri-
<br />ods of no flow and cannot be meaningfully analyzed for
<br />probability of non ex ceeda nee, Therefore, West Salt
<br />Creek, which had at lea~t 288 consecutive days of no
<br />flow recorded, is not shown, Low flows in high-
<br />altitude and mixed-type streams are sustained prima-
<br />rily by ground-water discharge, but gradual melting of
<br />perennial snowfields also provides some base flow,
<br />The shape of the annual minimwn mean 7-day stream-
<br />flow curves for the Colorado River below Baker Gulch
<br />and Colorado River near Cameo are similar, The
<br />greater magnitudes of low flow at Cameo for a specific
<br />probability ofnonexceedance arc a result ofa larger
<br />drainage area, tributary inflows, and water-dcvelop-
<br />ment factors such a~ reservoir releases, interba~in
<br />water transfers, and irrigation-return flows, At Cameo,
<br />7 consecutive days of flows less than 1,050 ft'/s can be
<br />expected 10 percent of the time, Knowledge about
<br />expected frequency of certain low flows is important
<br />because of the detrimental effect~ on stream biota
<br />resulting from dissolved-oxygen depletion and
<br />increased concentrations of dissolved constituents,
<br />
<br />Human Effects On Streamflow
<br />
<br />The natural hydrology of the Upper Colorado
<br />River Ba~in has been considerably altered by water
<br />development, which includes numerous reservoirs and
<br />diversions, The quantity of water removed from the
<br />basin by Iillg~ interba~in water transfers tD the SDulh
<br />Platte, Rio Grande, and Arkansas River Basins was
<br />about 585,000 acre-ft in water year 1993,
<br />
<br />20 Environmental Selling and Impllcatlona on Water Quality, Upper Colorado River Basin, Colorado and Utah
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