Opinion ID: 411109
Heading Depth: 3
Heading Rank: 1

Heading: Low Dissolved Oxygen

Text: 3 Water released from a reservoir through a dam into downstream water may be low in dissolved oxygen. The river below the dam will remain oxygen-depleted for some distance, although the river will gradually become reaerated through wind mixing as it flows downstream. 2 If the oxygen level is too low, fish cannot survive. Also, a river low in oxygen has limited ability to break down pollutants and other organic matter. Because dissolved oxygen is important both for fish and for breakdown of organic matter, it is an important measure of water quality. 3 4 Only large storage dams have low dissolved oxygen problems, and then only during warmer months and only when water is released from the lower part of the reservoir. 4 During warm months, deep reservoirs, like deep natural lakes, stratify into a cold, dense lower layer and a warmer, lighter upper layer. The upper layer, called the epilimnion, is aerated by wind mixing; oxygen is also produced by photosynthesis. Thus, water quality in the upper layer is good. The lower level, called the hypolimnion, is too deep to be aerated by wind action and light levels are too low to support photosynthesis. Organic decomposition, which consumes oxygen, leads to a continual net depletion of dissolved oxygen. Depletion continues until fall turnover, when the two layers break up and the reservoir returns to full aeration. 5 5 The rate of oxygen depletion depends primarily on the volume of water in the hypolimnion (the more water, the more oxygen is available for decomposition), its temperature (decomposition occurs more slowly in cold water and colder water also contains more dissolved oxygen), and the quantity of organic matter it contains (the more organic matter, the greater the oxygen demands for decomposition). In particular, if the river above the dam is high in plant nutrients or organic waste when it enters the reservoir--whether from pollution or from natural causes--oxygen depletion in the hypolimnion will be severe. 6 6 Several techniques can be used to prevent release of oxygen-depleted water. First, for many dams, water can be released from the epilimnion (which occurs automatically for natural lakes). Older dams were built with reservoir outlets at one level only, usually deep in the dam so that the outlet would remain below the surface of the reservoir even in dry years when the reservoir was low. Many newer dams, however, have outlets at several levels, permitting the dam operator to release high-quality epilimnion water. In single-outlet dams, one can aerate the reservoir (by pumping compressed air down to the hypolimnion) or destratify it (by pumping cold water from the hypolimnion to the surface). Alternatively, one can aerate the hypolimnion water as it is released from the reservoir, either by injecting air or by creating turbulence. 7 7 The record does not indicate the number of dams for which discharge of low-oxygen water is a significant problem, nor the cost of the various methods of mechanical aeration. 8 But the problem is serious for at least some dams, and the cure is apparently expensive. 9