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Total-nitrogen concentrations were less than 2.0 milligrams per liter, and total-phosphorus concentrations ranged from 0.062 to 0.148 milli- gram per liter. Dissolved-solids concentrations ranged from 1,870 to 2,370 milligrams per liter. and sodium and sulfate comprised the largest per- centage of the major chemical constituents. Con- centrations of total-recoverable boron and concen- trations of total-recoverable iron occasionally exceeded the water-quality standards set for Lake Meredith Reservoir by the Colorado Department of Health. Concentrations of other trace elements were less than the State water-quality standards for Lake Meredith Reservoir or less than the detection limits for the analysis. Phytoplankton densities in Lake Meredith Reservoir ranged from 58,000 to 3,900,000 cells per milliliter. Blue-green algae accounted for the largest percentage of the phytoplankton density. The trophic status of Lake Meredith Reservoir is eutrophic as indicated by the trophic-state index and the fixed-boundary system. INTRODUCTION Lakes and reservoirs in the lower Arkansas River basin in southeastern Colorado historically have been used primarily for storage of irrigation water and for ftood control. During the last decade, numerous changes and anticipated changes in land use and in water operations and use in the basin have combined to increase the importance of lake and reservoir water quality. Because the quality of water of lakes and res- ervoirs can impair water use, there is a need to assess the existing waler quality and trophic status of natural lakes and reservoirs in southeastern Colorado. This information is needed to evaluate the suitability of these lakes and reservoirs for existing uses and to serve as a baseline for comparison as changes occur in the basin. However, water quality has not been a major concern in the past. and minimal water-quality data are available for lakes and reservoirs in the area. Addi- tional data are needed for making sound management decisions relative to changes in water use and manage- ment and for establishing appropriate in-stream water- quality standards because, as changes in water use and management occur within the basin, the resulting changes in slreamftow may affect water quality. There- fore, a study was begun in ] 987 by the U.S. Geological Survey. in cooperation with the Southeastern Colorado Water Conservancy District, to determine water- quality conditions of two selected reservoirs in the lower Arkansas River basin that will be used as part of water exchanges by the city of Colorado Springs. Spe- cific objectives of the study were to: (]) Assess the physical, chemical, and biological quality of Lake Henry and Lake Meredith Reservoir; (2) compare physical properties and results of analyses for chemical constituents of the water in these reservoirs with water- quality standards established by the Colorado Depart- ment of Health (1987); (3) estimate present trophic sta- tus of each reservoir; and (4) compare the water quality between the two reservoirs. Both reservoirs are used as wildlife refuges and provide an important fishery in southeastern Colorado. Purpose and Scope This report describes the water-quality charac- teristics of Lake Henry and Lake Meredith Reservoir and the comparison between the characteristics and the water-quality standards established for these reser- voirs. Lake Meredith Reservoir is locally referred to as Lake Meredith and is hereinafter referred to as Lake Meredith. Measurements of water temperature, dis- solved oxygen, pH, specific conductance, and light transparency are reported for numerous sites in each lake. In addition, analyses of samples collected for nutrients (nitrogen and phosphorus). major chemica] constituents, trace elements, phytoplankton densities, and chlorophyll a concentrations in Lake Henry and Lake Meredith are included in this report. The report includes a description of some of the water-quality characteristics of each lake, compares the current water quality of Lake Henry and Lake Meredith to the water- quality standards established by the Colorado Depart- ment of Hea]th (] 987), describes the trophic status of each lake, and compares the water quality between the two lakes. Bathymetric measurements were made at both lakes in April. The onsite measurements and co]- lection of water samples for chemical and biological analyses were done in May, June, August, and October ]987. Description of Lake Henry and Lake Meredith Lake Henry and Lake Meredith are located in Crowley County in southeastern Colorado in an agri- cultural area of the Arkansas River valley. Lake Henry is about 3 mi northeast of the city of Ordway and Lake Meredith is about 2 mi southeast of the city of Ordway (fig. I). Both lakes are primarily used as storage reser- voirs for irrigation water and water exchanges for the Colorado Canal. Other uses of the lakes include boat- ing and fishing. The city of Colorado Springs is the 2 Reconnaissance 01 Water Quality 01 Lake Henry and Lake Meredith Reservoir, Crowley County, Southeastern Colorado, April-October 1987