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<br />~ <br />o <br />~ <br />CJI <br /> <br />about 25 miles (40 km) above the Grand Canyon gaging station <br />(Figure 1), Flows in the Little Colorado are derived from <br />surface runoff, which is highly variable seasonally, and a <br />nearly constant base flow of 223 CFS (6.3 m3/sec) from sev- <br />eral springs and seeps, collectively referred to as Blue <br />Springs [26J. Blue Springs contribute about 550,000 tons of <br />salt per year to the Colorado River [4J. Comparison of flow- <br />weighted average ion concentrations during the 1970-79 peri- <br />od at Lees Ferry and Grand Canyon (Table III) indicate that <br />the Blue Springs input is comprised primarily of sodium and <br />chloride. <br /> <br />Table III. Flow-weighted Average Ion and TDS Concentra- <br />tion at Lees Ferry and Grand Canyon During the <br />1970-79 Period. [USGS dataJ. <br />Constituents (mg/l) <br />-Lo('1'tion Ca Mg Na K C03 SO" Cl Si *TDS <br /> <br />Leflll Ferry 72 25 <br />Grand Canyon 75 26 <br /> <br />76 3.9 79 <br />96 4.1 84 <br /> <br />238 52 <br />243 81 <br /> <br />554 <br />618 <br /> <br />8.2 <br />8.5 <br /> <br />~ 3 1 20 0.2 <br />*Sum of constituents <br /> <br />5 <br /> <br />5 29 0.3 <br /> <br />64 <br /> <br />Concentrations of these ions in Grand Canyon will vary <br />inversely with river flows. Glen Canyon Dam stabilized sea- <br />sonal flows but resulted in extreme variations in hourly <br />flows. It is common for daily flows to vary from 2000 CFS <br />(57 m3/sec) to over 20,000 CFS (566 m3/sec). USGS sampling <br />in Grand Canyon has almost always been conducted at the be- <br />ginning of each month, but the actual time of sampling var- <br />ies from month to month. Sampling conducted when flows are <br />high will cause more dilution of sodium and chloride concen- <br />trations and result in underestimates of loading to Lake <br />Mead. This appears to be the case during the 1970-79 period <br />which accounts for the unusual discrepancy observed in the <br />sodium and chloride budgets, <br />This illustrates the kind of problems that can develop <br />where sampling is conducted below a point source tributary <br />in a regulated river. Grand Canyon is an extreme case due to <br />the unique nature of the Little Colorado River, but a simi- <br />lar situation could exist in the Green River, which is regu- <br />lated by Flaming Gorge Dam. The USGS gaging station in the <br />Green River at Green River, Utah is located below the Price <br />River which is a significant TDS point source [4J. At these <br />locations, it seems that composite, rather than grab, sam- <br />pling should be conducted to insure that flow-induced vari- <br />ations in ion concentrations are adequately represented by <br />the sample. <br /> <br />13 <br />