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WATER-QUALITY CHARACTERISTICS OF LAKE MEREDITH AND COMPARISON TO WATER-QUALITY STANDARDS Water-quality characteristics of Lake Meredith were determined by onsite measurements made at 10 sites in the lake and by laboratory analysis for seJected chemical and biological constituents in water samples collected at sites M2B, M I B, and M4B (fig. 3). PhysicaJ properties and results of analyses for chemical constituents were compared with water- quaJity standards established by the Colorado Depart- ment of Health. Onsite Measurements The water-temperature profiles shown in figure 8 are representative of the thermal characteristics that existed throughout Lake Meredith on the sampling dates. The temperatures in the lake ranged from J5.30C at sites M2B and MIB near the lake bottom on May J to 27.90C at site MIC at the water surface on August 20 (table 18). Temperature measurements on May J indicate that the lake was thermally stratified because of spring warming of the upper water layer. The difference in the profiJe curves between site M2B and sites M I Band M4B on May I probably was due to mixing of the upper water layers from wave action caused by wind. On June 30, no thermaJ stratification occurred, and temperature was uniform vertically and areally throughout the lake. On August 20 and 21 and October 6, water temperatures decreased with depth. The dissolved-oxygen and pH profiles shown in figure 9 exhibited similar changes with depth indicat- ing dissolved-oxygen concentrations and pH are affected by photosynthesis and respiration. Dissolved-oxygen concentrations in Lake Meredith ranged from 1.4 mg/L at site M I A at the lake bollom on August 20 to 10.3 mgIL at site M2B at a depth of 3 ft on May I. On May I. Lake Meredith was thermally stratified, and a density barrier prevented transfer of dissolved oxygen from the surface waters to the lake bottom resulting in a stratification of dissolved oxygen. The increase in the dissolved-oxygen concen- tration from the Jake surface to a depth of about 3 ft at site M2B on May I probably was due to a concentra- tion of phytoplankton at the top of the density barrier caused by thermal stratification. The larger dissolved- oxygen concentrations on May 1 probably were a result of photosynthesis and cooler water temperatures. On June 30 and August 20 and 21, warmer watertempera- tures contributed to algal blooms, which then caused a depletion of dissolved oxygen near the lake bottom because of decomposing aJgae. On October 6, dissoJved-oxygen concentrations were larger than those on June 30 and August 20 and 21 because of cooJer water temperatures. A minimum allowable dissoJved-oxygen con- centration of 5.0 mgIL has been established for Lake Meredith for the maintenance of aquatic life and agri. culture by the Colorado Department of Health (1987). Twenty measurements of dissolved-oxygen concentra- tions less than the 5.0 mgIL standard were made at lower depths. A summary of dissolved-oxygen mea- surements including those that were less than 5.0 mgIL are listed in table 8. The pH of the water in Lake Meredith is affected by water that enters the lake from the Lake Meredith inlet and Bob Creek and by photosynthesis and respira- tion (table 9). The pH of the water in the lake ranged from 7.9 to 8.7 during the sampling period. Larger pH values occurred at the lake surface because of photo- synthesis, and smaller pH occurred near the lake bot- tom where respiration predominates. The allowable range of pH established for Lake Meredith by the Col- orado Department of Health (1987) for aquatic life is 6.5 to 9.0, and the measured pH in the lake during the sampling period was within this range. Profiles of specific conductance indicate there was minimal vertical and areal variation in measured values (fig. 10). Specific conductance measured during the sampling period ranged from 2,520 IlS/cm on June 30 to 3,040 IlS/cm on August 21. Between the June and August sampling dates, an increase in mean specific conductance of about 13 percent (from 2.560 to 2,950 IlS/cm) occurred in the lake. The temporal variations in specific conductance measured in Lake Meredith probably are the result of changes in the spe- cific conductance of the inflows and an increase in the concentration of dissolved solids due to evaporation. Light transparency in Lake Meredith probably is affected most by phytoplankton biomass; however, suspended sediments from the inflows and disturbed bottom sediments from wave action could decrease light transparency. Secchi-disk measurements varied areally and temporally in Lake Meredith. Secchi-disk depths generally were greater on May I than on the other sampling dates. The Secchi-disk depths were either 1.0 or 1.5 ft on May 1 and either 0.5 or 1.0 ft dur- ing the remainder of the sampling period (table 10). The areal variations in light transparency probably are due to a nonuniform distribution of phytoplankton within the lake. 20 Reconnaissance of Water Quality of Lake Henry and Lake Meredith Reservoir, Crowley County, Southeastern Colorado, April-October 1987