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,-. r;.1 'J- t.o'....:. Although sediment loads to Pueblo Reservoir are greatly decreased during the fall relative to the summer, and algal biomass generally was smaller during the fall than during the summer, Secchi-disk measurements remained small. Algal biomass seems to be the domi- nant effect on water transparency during the fall. Dur- ing the fall, the depth of the euphotic zone generally is less than about 2 m in the upstream part of the reservoir as shown by site 3B in figure 8, less than about 3 m in the middle part of the reservoir as shown by site 5C, and less than about 3 m in the downstream part of the reservoir as shown by site 78. The persistence of low- water transparency during the fall might partly be caused by fall turnover resuspending algae and detritus into the euphotic zone that concentrated in the metalim- nion during the summer or from fall blooms of phy- toplankton, or both, Distribution and Transport of Particulate Matter A semiquantitative method of evaluating distri- bution and transport of particulate matter in Pueblo Reservoir was made using numerous turbidity mea- surements made at multiple depths (3-5) and sites in the reservoir (fig, 9). Turbidity is a measurement of the clarity of water and is affected by suspended matter such as clay; silt; finely divided organic matter; soluble, colored, organic compounds; plankton; and other microscopic organisms (American Public Health Asso- ciation and others, 1985). Although turbidity is an expression of the optical property of water rather than a measurement of the actual suspended-sediment con- centration, a comparison of turbidity measurements with concurrent suspended-sediment concentration measurements made from samples collected from the Arkansas River upstream from Pueblo Reservoir (station 07097000, Arkansas River at Portland) indi- cate a highly significant correlation (.-2 of 0.91 at a sig- nificance level of 0.000 I) between turbidity and suspended-sediment concentration. The predominant sources of suspended material in Pueblo Reservoir are the Arkansas River drainage basin and the production of plankton within the reser- voir. Other potential sources of suspended material include resuspension, shoreline erosion, and tributary inflow. The larger size fraction (sand) of suspended sediment within the reservoir settles from the water column relatively quickly. However, the finer size frac- tions of sediments (clay, silt, and, especially, colloidal material) and organic particulate matter can be trans- ported farther into the reservoir. The predominant spatial and temporal distribu- tion patterns and transport of particulates in Pueblo Reservoir can be described by using the December 1986 and March through October 1987 turbidity data presented in figure 9. The majority of the suspended sediment entering Pueblo Reservoir settles out of the water column within about 5 mi from where the river enters the reservoir which, during 1986 and 1987, cor- responds to an area between transects 3 and 4, From October through March, turbidity data collected from Pueblo Reservoir indicate a relatively small amount of suspended matter is present from the Arkansas River during this period. An increase in suspended sediment to Pueblo Reservoir occurs in April and May because of snowmelt runoff from the lower elevations in the upper basin, which results in an increase in particulates throughout the water column in the upstream 5 mi of the reservoir. Most of these sediments settle from the water column within about 3 mi from where the river enters the reservoir which, during 1987, corresponded to an area between transects 2 and 3. The remaining sediments are transported about 2 more miles prior to settling out. The turbidity profiles for June through September (fig. 9) are similar to the specific- conductance profiles for June through September (fig. 5) and indicate the distribution and transport of particulates in Pueblo Reservoir primarily is affected by the Arkansas River. During June, the peak snow- melt runoff in the upper basin occurs, which results in a large sediment load to the reservoir and a short hydraulic residence time of inflowing water as large volumes of water are released from the reservoir. During this period, underflow also is predominant. Although most particulates settle from the water col- umn within 3 mi from the river inflow, some particu- lates are transported almost 8 mi into the reservoir which, during 1987, corresponds to transect 6. As sum- mer progresses, underflow continues to predominate, and hydraulic residence time continues to be short. During this period, most of the suspended material seems to be transported with the underflow and settles from the water column within the upstream 5 mi of the reservoir. However, some particulates are transported through the entire reservoir, CHEMICAL CHARACTERISTICS The chemical processes that occur in Pueblo Reservoir are complex and interrelated with the physi- cal and biological processes. Because of these rela- tions, the chemical characteristics of Pueblo Reservoir vary spatially and seasonally. Water-quality data col- lected during 1986 through 1989 from Pueblo Reser- voir are used to describe the areal, vertical, and CHEMICAL CHARACTERISTICS 21