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<br />~~~i <br /> <br />and December, specific conductance at transects 3 and 5 markedly increased <br />below the thermocline, indicating that concentrations of dissolved solids were <br />much larger in the lower strata near the reservoir bottom, probably as the <br />result of river water underflow that had a greater specific conductance than <br />the reservoir. Specific-conductance measurements at transect 7 were the same <br />throughout the water column during September and December. This indicates <br />that the reservoir was thoroughly mixed near the outflow, possibly as a result <br />of induced mixing from the outflow. <br /> <br />Monthly variations in specific conductance measured at the reservoir sur- <br />face and reservoir bottom at transects 1, 3, 5, and 7 are shown in figure 11 <br />and indicate that specific conductance increased throughout the reservoir from <br />June through December. At transect 7, the monthly variations are less pro- <br />nounced than at transects 1, 3, and 5, possibly as a result of withdrawals <br />from the dam that induce mixing. The variations in specific conductance that <br />occurred in the reservoir generally coincided with variations in specific <br />conductance measured at station 07097000, Arkansas River at Portland (fig. 6), <br />and indicate that concentrations of dissolved solids in the reservoir are <br />affected largely by dissolved-solids concentration in the Arkansas River <br />upstream from the reservoir. <br /> <br />Light Transparency <br /> <br />Light transparency is the capability of water to transmit light and <br />determines the depths where sufficient light exists for photosynthesis to <br />occur. The euphotic zone is the area of the lake where light penetration is <br />sufficient for photosynthesis. An approximation of the capability of water in <br />Pueblo Reservoir to transmit light was made using a Secchi disk. The measure- <br />ment consists of recording the depth at which the disk disappears from view. <br />Secchi-disk depth is a function of the reflection of light from its surface <br />and is affected by surface disturbance (waves), absorption characteristics of <br />the water, color of the water, and dissolved and particulate matter in the <br />water. The Secchi-disk depth correlates closely with percentage of light <br />transmission. At the extremes, Secchi-disk depth can represent from 1- to <br />IS-percent light transmission (Wetzel, 1983). Hutchinson (1957) noted that <br />the Secchi disk disappears from view at about the light-penetration level of <br />5 percent of surface light. The compensation level, or the depth of water at <br />which oxygen production by photosynthesis equals the oxygen consumption by <br />respiration, usually occurs when light intensity is decreased to about 1 per- <br />cent of surface light. The depth of the euphotic zone in Pueblo Reservoir is <br />estimated to be about 2 to 3 times the Secchi-disk depth. <br /> <br />Secchi-disk depths ranged from less than 1 ft at transect 1, located near <br />the inflow, to 9.5 ft at transect 6 (fig. 12). In general, Secchi-disk depths <br />increased from transect 1 to transect 7, indicating a possible decrease in <br />suspended sediment and algal biomass from transect 1 to transect 7. During <br />September and October, anomalously small Secchi-disk depths were measured, <br />possibly due to a greater occurrence of cloud cover. <br /> <br />27 <br />