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<br />~t.0 <br />2~~~~ <br /> <br />WATER-QUALITY CHARACTERISTICS OF PUEBLO RESERVOIR <br /> <br />The physical, chemical, and biological processes within lakes and reser- <br />voirs are complex and interrelated. In order to better understand the water- <br />quality characteristics of Pueblo Reservoir, the physical, chemical, and <br />biological constituents measured during the summer of 1985 are discussed in <br />the following sections of this report. <br /> <br />Onsite Water-Quality Measurements <br /> <br />The properties and constituents measured onsite during the summer of 1985 <br />were water temperature, specific conductance, light transparency, dissolved <br />oxygen, and pH. The areal, vertical, and seasonal variations of ansite water- <br />quality measurements are described in the following sections. <br /> <br />Water Temperature <br /> <br />Water temperature is one of the most important environmental properties <br />of a lake because life processes, chemical reactions, and the solubility of <br />chemical constituents in water are temperature dependent. For example, the <br />solubility of dissolved oxygen is inversely related to temperature; that is, <br />the warmer the water the less oxygen can be dissolved in the water. This <br />relation is significant, because at warmer temperatures organisms have an <br />increased metabolic rate but have less oxygen available for their physiolog- <br />ical needs. <br /> <br />Water temperature also is a major factor in controlling the density of <br />freshwater. Freshwater is unique because its maximum density occurs at about <br />4 DC. The density of water decreases as water temperatures vary from 4 oC. <br />Conversely, the density of fresh water increases as water temperatures <br />approach 4 oC. Density also is affected by salinity. Density increases with <br />increasing concentrations of dissolved solids in an approximately linear <br />fashion (Wetzel, 1983). Differences in density inhibit mixing of waters of <br />differing temperatures and salinity and can result in stratification of the <br />water layers. Stratification is a condition whereby less dense water overlies <br />more dense water. Water-temperature measurements are used to determine the <br />depths where various layers of water occur that possibly have different chem- <br />ical characteristics. When the lake is stratified, the upper water layer or <br />the epi1imnion is thermally uniform and contains the warmest water in the <br />lake. Between the epilimnion and hypolimnion is the middle water layer or <br />metalimnion where water temperature rapidly decreases with depth. The thermo- <br />cline as defined by Wetzel (1983) is the plane of maximum rate of decrease of <br />temperature with depth. The lower water layer containing the coldest water is <br />the hypolimnion. <br /> <br />Selected temperature profiles measured in Pueblo Reservoir during 1985 <br />are shown in figure 8. Temperature profiles measured at transects 3, 5, and 7 <br />during June 1985 indicate that there were multiple water layers and that the <br />reservoir was complexly stratified. As air temperature decreased in the <br />autumn, the upper water layers cooled and mixed, but the lower water layers <br />near the bottom of the reservoir remained stratified; this is indicated by <br /> <br />22 <br />