The principal effects of impoundment upon river biota affect management practices. Water released from an impoundment into a receiving river usually is by overflow at the dam or discharge from an outlet at depth. The thickness of the water layer involved in this outflow is controlled by density stratification: the stronger the density gradient, the smaller the thickness of the layer of homogeneous density which is drawn off, but the greater must be its areal extent. The decisive variable for the thickness of the outflow layer is flow rate in the center of the outflow layer, owing to the pressure of the overlying water (determined mainly by the depth) and the size of the opening. In this way, the influence of density stratification within an impoundment is important not just for the temperature of the outflowing water, but also for its chemical and biological quality.
The characteristics of outflow water differ between shallow and deep impoundments. The quality of outflow from a shallow, homogeneously mixed impoundment is likely to be similar to that of its inflow or, if not, to be poorer rather than improved. The concentration of bio-available phosphate is not much altered within shallow basins, despite high primary production, because frequent resuspension of bottom sediment speeds the rate of phosphorus recycling. On the other hand, presently incomplete evaluations suggest that nitrogen, especially nitrate, may be significantly decreased. Because primary production is vertically confined and there can be relatively low light absorption in short water columns that do not suffer from high inorganic turbidity, the concentration of phytoplankton in shallow basins is potentially much greater than that in unstratified deep impoundments.
Strong density stratification in deep impoundments may promote the formation of local depth maxima of phytoplankton but the interaction of stratification and long retention times produces other significant effects on the potential quality of the outflow. Surface water may benefit from precipitation of clays and other particles, but it may become enriched by phytoplankton. Drawoff from the hypolimnion of a productive impoundment is liable to be deficient in oxygen, malodorous and liable to precipitate iron and manganese salts in the outflow. However, the characteristics of water in an outflow layer are not constant over time because the exported water is replaced, ultimately, from other layers. At the very least, there will be a direct gravitational substitution of water from the erstwhile more superficial layers. This will be compounded by associated turbulence. Potentially, the horizontal intrusion of cold inflow to its density-determined depth can cut short the volumetrically determined storage time by finding its way, substantially intact, to the drawoff point.
Temperature of the outflow reflects the geophysical site conditions, compounded by the hydraulic retentivity. If the outflow is located in the hypolimnion, the temperature of the outflowing water will be intermediate between that of the impoundment at its surface and at the drawoff depth. During seasonal mixing, however, the temperatures at all depths are similar. Oxygen conditions in the outflow depend on the balance between production and respiration processes in the contributing water layers, subject to any equilibration that may take place in the turbines or other machinery. Surface release water can be supersaturated with oxygen as a result of intense photosynthesis; at worst, deep drawoff can be anoxic. Organic pollution invariably contributes to lowered oxygen concentrations.
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