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0061 Table 14. Trophic-state indices for sites M2B, M1B, and M4B at Lake Meredith ITSI, trophic-Slate index: SD, Secchi-disk depth; TP. total phosphorus; ChI. chlorophyll oj Sampling Blta TSI TSI TSI (1Ig.3) (SO) (TP) (Chi) M2B 87 70 50 MIB 82 72 52 M4B 87 76 56 Average (all sites) 85 73 53 Lake Meredith is hypereutrophic with respect to Secchi- disk depth and total-phosphorus concentration and eutrophic with respect to chlorophyll a concentration. COMPARISON OF SELECTED WATER- QUALITY CHARACTERISTICS BETWEEN LAKE HENRY AND LAKE MEREDITH Water released from Lake Henry can be diverted into one of the inflows to Lake Meredith (fig. I); there- fore, the water quality of Lake Henry may have an effect on the water quality of Lake Meredith. During the sampling period, the thennal properties of Lake Henry and Lake Meredith were similar. Both lakes were thennally stratified on the May sampling date and not stratified on the June sampling date. and temper- ature profiles of both lakes were similar on the August and October sampling dates. Dissolved-oxygen concen- trations and pH values in Lake Henry and in Lake Meredith were larger near the lake surface and were smaller near the lake bottom because of the effect of pho- tosynthesis and respiration by phytoplankton. Average specific conductance in Lake Henry was about one-third the average specific conductance in Lake Meredith indi- cating that Lake Henry had smaller dissolved-solids con- centrations than did Lake Meredith. Nitrogen and phosphorus concentrations varied between Lake Henry and Lake Meredith. Concentrations of total nitrogen averaged about I. I mg/L in Lake Henry and less than about 1.6 mgIL in Lake Meredith. Concen- trations of total phosphorus averaged about 0.066 mgIL in Lake Henry and about 0.106 mgIL in Lake Meredith. The ratios of dissolved-inorganic nitrogen to dissolved orthophosphate as phosphorus for Lake Henry and Lake Meredith indicate that phosphorus has the potential of being the limiting plant nutrient in both lakes. However, the large algal biomass sampled in each lake indicates neither nitrogen nor phosphorus are limiting nutri- ents. The predominant chemical constituents were calcium and sulfate in Lake Henry and sodium and sulfate in Lake Meredith. The water in both lakes had a small sodium-adsorption ratio. However. the sodium-adsorption ratio values for Lake Meredith (average about 5) were substantially larger than for Lake Henry (average about 1.4). Concentrations of total-recoverable iron in Lake Henry and total- recoverable iron and total-recoverable boron in Lake Meredith were the only chemical constituents that occasionally exceeded the water-quality stan- dards established by the Colorado Department of Health (1987). The larger concentrations of total- recoverable iron and total-recoverable boron may have been caused by reducing conditions in the lower depths of the lakes under depleted dissolved- oxygen concentrations or concentration byevapo- ration. The average phytoplankton densities in Lake Henry were about 38 percent of the average phy- toplankton densities in Lake Meredith. During the sampling period. blue-green algae comprised the largest percentage of the phytoplankton densities in both lakes. The trophic status of both lakes would be considered eutrophic using trophic-clas- sification systems developed by Carlson (1977) and the Organisation for Economic Co-operation and Development (1982); however, Lake Meredith could be considered more eutrophic than Lake Henry. SUMMARY Lake Henry and Lake Meredith are shallow, off-stream storage reservoirs for the Colorado Canal. Water stored in each lake primarily is used for agricultural purposes. The Colorado Depart- ment of Health has established water-quality stan- dards pertaining to the uses of both lakes. Lake Henry and Lake Meredith were sam- pled four times during 1987 for the purposes of; (I) Evaluating selected water-quality characteris- tics of each lake; (2) comparing the current water quality of Lake Henry and Lake Meredith to water- quality standards established by the Colorado Department of Health (1987); (3) describing the trophic status of each lake; and (4) comparing the water-quality between the two lakes. During the study period. water temperature in Lake Henry ranged from 14.2 to 26.40C. The lake was thennally stratified on May I and not stratified on June 29. Concentrations of dissolved oxygen ranged from 4.7 to 7.8 mgIL; the pH COMPARISON OF SELECTED WATER-OUALlTY CHARACTERISTICS BETWEEN LAKE HENRY AND LAKE MEREDITH 29