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OJ." ') - ... ()t". ~) Physical, Chemical, and Biological Characteristics of Pueblo Reservoir, Colorado, 1985-89 ByMichael E. Lewis and Patrick Edelmann Abstract Physical, chemical, and biological charac- teristics of Pueblo Reservoir are described on the basis of data collected from spring 1985 through fall 1989. Also included are discussions of water quality of the upper Arkansas River Basin and the reservoir as they relate to reservoir operations. Pueblo Reservoir is a multipurpose, main-stem reservoir on the Arkansas River about 6 miles west of Pueblo. Colorado. At the top of its conservation pool. the reservoir is more than 9 miles long and ranges in depth from a few feet at the inflow to about 155 feet at the dam. Pueblo Reservoir derives most of its contents from the Arkansas River, which comprises native and transmountain flow, With respect to water temperature, the reser- voir typically was well mixed to weakly stratified during the early spring and gradually became strongly stratified by May. The strong thermal stratification and underflow of the Arkansas River generally persisted into August, at which time the reservoir surface began to cool and the reservoir subsequently underwent fall turnover. Following fall turnover, the reservoir was stratified to some degree in the shallow upstream part and well mixed in the deeper middle and downstream parts. Reservoir residence times were affected by the extent of stratification present. When the reservoir was well mixed, residence times were as long as several months. During the summer when the res- ervoir was strongly stratified. reservoir releases were large, and when underflow was the prevalent flow pattern of the Arkansas River. reservoir resi- dence times were as short as 30 days. Most particulate matter settled from the water colunm between the inflow and a distance of about 5 miles downstream. On occasions of large streamllows and sediment loads from the Arkan- sas River, particulate matter was transported com- pletely through the reservoir. Water transparency, as measured with a Secchi disk, increased in a downstream direction from the reservoir inflow. The increase probably was a result of sediment set- tling from the water column in the upstream part of the reservoir, Secchi-disk depths in December through April were larger than those in May through November. Secchi-disk depths were small between May through August as inflow sed- iment loads and reservoir biomass increased. In the fall. Secchi-disk depths remained small possi- bly as the result of resuspension of sediment and detritus within the water column. Dissolved-oxygen concentrations generally were near supersaturation near the reservoir sur- face, Dissolved-oxygen concentrations decreased with increasing depth. On several occasions dur- ing the summer, dissolved oxygen became com- pletely depleted in the hypolimnion of the downstream part of the reservoir. The most exten- sive period of anoxia that was measured was in August 1988; the bottom 12 to 30 feet of the down- stream end of the reservoir was anoxic. Fall turn- over typically resulted in well-oxygenated conditions throughout the water column from September or October through the spring, Values of pH ranged from 7.5 to 9,0 and typically were largest near the surface and decreased with depth. Dissolved-solids concentrations in the res- ervoir primarily are affected by dissolved solids in the inflow from the Arkansas River. Concentra- tions are largest during periods of decreased streamflows, September through April, and decrease with increasing stream flows in May through August. The median dissolved-solids concentration increased from 224 milligrams per liter at the inflow to 262 milligrams per liter at the outflow, However. a statistical analysis of dis- sol ved solids indicated the apparent increase in dissolved-solids concentrations between the inflow and outflow was not significant. Calcium, Abatract