|
1,000,000
<br />F- 0
<br />ow
<br />LL w -1,000,000
<br />2 cc
<br />W U -2,000,000
<br />LUQ
<br />F Z -3,000,000
<br />(n -
<br />-4,000,000
<br />1,000,000
<br />oU)
<br />J Z
<br />00
<br />(n 0
<br />0 Z
<br />LU
<br />o Q -1,000,000
<br />(n o
<br />(n J
<br />o -2,000,000
<br />U) 500
<br />0Z(n
<br />_J o2 250
<br />oP: QCC
<br />occ (Dcc LU
<br />~ 0
<br />W H J J
<br />?ZJM 250
<br />U) Z d -500
<br />(n o ?
<br />0 U 750 1 *1
<br />0 N D J F M A M J J A S ANNUAL
<br />MONTH
<br />EXPLANATION
<br />Highly significant (p 50.01)
<br />® Significant (0.01< P!0.05)
<br />® Marginally significant (0.05<p!50.10)
<br />Figure 26. Step trends at site 69 (Colorado River at Lees Ferry,
<br />Ariz.) from 1942-62 to 1966-80.
<br />tration during 1948-50 was 1,340 mg/L (table 9); sulfate
<br />was 61 percent of the dissolved-solids load. Unlike the Dirty
<br />Devil River basin, the Paria River basin does not contain
<br />large outcrops of the Carmel Formation, and the proportion
<br />of chloride is much lower, about 2 percent of the dissolved-
<br />solids load. Calcium, sodium, and sulfate are the predomi-
<br />nant ions throughout the year. Annual dissolved-solids load
<br />from 1948 to 1950 averaged 32,000 tons (table 9).
<br />General Trends in the San Juan Region
<br />Annual streamflow of the San Juan River has not
<br />changed appreciably. Streamflow of the Colorado River at
<br />site 69 at Lees Ferry, Ariz., essentially represents the outflow
<br />of the entire Upper Colorado River Basin. Streamflow at
<br />Lees Ferry indicates changes throughout the basin, as well
<br />as the storage of inflows in Lake Powell and the complete
<br />regulation of outflows from Glen Canyon Reservoir.
<br />Annual step-trend analyses at sites along the San Juan
<br />River indicate a decrease in monthly variability but little
<br />change in annual streamflow or dissolved solids. A marginal-
<br />ly significant increase in dissolved-solids concentration at site
<br />68 indicates that a change occurred during the filling of
<br />Navajo Reservoir.
<br />Annual step-trend analyses for the Colorado River at
<br />site 69 at Lees Ferry, Ariz., indicate the decrease in stream-
<br />flow and dissolved-solids load during the principal filling
<br />period of Lake Powell, as well as the virtual elimination of
<br />seasonal variability. Annual monotonic-trend analyses indi-
<br />cated that dissolved-solids concentration increased during the
<br />preintervention period, possibly resulting from increased
<br />development and exports upstream. Since 1942-62, however,
<br />dissolved-solids concentration has not changed significantly.
<br />Lake Powell and Dissolved-Solids Outflow
<br />from the Upper Colorado River Basin
<br />Because of its size and location, Lake Powell is the
<br />single most important anthropogenic feature affecting
<br />streamflow and dissolved solids in the Upper Colorado River
<br />Basin. Because of its capacity to store and mix inflows, the
<br />reservoir has decreased the seasonal and annual variability of
<br />streamflow and dissolved solids downstream from Glen
<br />Canyon Dam. Located at the downstream end of the Upper
<br />Colorado River Basin, releases from the reservoir effectively
<br />control the volume of water supplied to the Lower Colorado
<br />River Basin.
<br />In terms of its effects on dissolved solids, Lake Powell
<br />primarily is a large mixing tank. After the initial filling
<br />period, the monthly dissolved-solids concentration in the
<br />outflow water stabilized within a narrow range (fig: 27). Dur-
<br />ing water years 1971-83, monthly dissolved-solids concen-
<br />trations in the outflow (site 69) ranged from 492 to 645 mg/L.
<br />Within that same period, concentrations in the inflow to Lake
<br />Powell (flow-weighted mean of sites 21, 54, 57, and 68)
<br />ranged from 261 to 904 mg/L, more than four times the range
<br />of the outflow concentration.
<br />The mixing-tank concept was tested using a simple
<br />mass-balance procedure. Monthly inflow volumes and
<br />dissolved-solids loads were computed by summation of the
<br />streamflows and loads at sites 21, 54, 57, and 68. The
<br />measured volumetric storage change for Lake Powell was
<br />compared to the difference between the inflow and outflow
<br />volumes during the month. The dissolved-solids mass in
<br />storage was calculated as the product of the volume in storage
<br />(V„,) and the mean monthly outflow concentration at site 69
<br />(Cm). Mass-balance equations could then be written for the
<br />change in volume:
<br />V.- Vm-1 = Q I, m - QO, m + UV' M (6)
<br />and the change in dissolved-solids load:
<br />C.Vm-Cm-1Vm-1=Ll,m-Lo,m+UL,m (7)
<br />where
<br />Vm = the volume in storage at the end of month m;
<br />V„ 1- I = the volume in storage at the end of the previous
<br />month (m -1);
<br />QI,m = the total inflow volume during month m;
<br />58 Characteristics and Trends of Streamflow and Dissolved Solids in the Colorado River Basin
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