Opinion ID: 411109
Heading Depth: 2
Heading Rank: 1

Heading: Dam-Induced Water Quality Changes

Text: 2 Dams cause a variety of interrelated water quality problems, both in reservoirs and in river water downstream from a dam. The Wildlife Federation claims that five of these problems--low dissolved oxygen, dissolved minerals and nutrients, water temperature changes, sediment release, and supersaturation--require EPA to regulate dams under the Sec. 402 permit program. The district court's opinion, 530 F.Supp. at 1297-1303, describes these problems in detail, and we will only summarize them here. 1
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
8 If dissolved oxygen is totally depleted from the hypolimnion, a further problem develops. A number of minerals and plant nutrients, insoluble under normal aerobic conditions, are soluble in zero-oxygen anaerobic water. These compounds--including iron, manganese, and phosphates--therefore tend to be leached from bottom muds into the reservoir. High concentrations of these minerals and nutrients, released into the downstream river, can harm fish, make the water unpalatable for drinking, and foster undesirable plant growth. 10 9 As for low-dissolved oxygen problems generally, whether mineral leaching will occur depends on a number of factors, including reservoir size, water temperature, and the quality of upstream water. In addition, mineral leaching depends on the amount of leachable matter in the reservoir bottom, which in turn depends partly on how old the reservoir is (for an older reservoir, most leachable minerals may have already been leached). 11 10 Control of mineral leaching primarily involves destratifying or mechanically aerating the reservoir to prevent the hypolimnion from becoming totally depleted, or else discharging water from the epilimnion. When building a new dam, site preparation (e.g., removing organic soils) can reduce future leaching. Once again, the record reveals neither the number of dams for which mineral leaching is a significant problem nor the cost of cure.
11 In a thermally stratified reservoir, the lower hypolimnion layer will generally be colder than the upstream river, while the upper epilimnion layer will be warmer. Some species of fish can survive only in warm water; others can survive only in cold water. Thus, cold hypolimnion water, even if fully oxygenated, will harm or kill warm water fish but benefit cold water fish; conversely, warm epilimnion water will harm or kill cold water fish and benefit warm water fish. In some cases, cold water discharges may be desirable--to create a trout fishery, for example. 12 Also, colder water has higher capacity to assimilate wastes, both because decomposition is slower and because oxygen is more soluble in cold water. 13 In short, dams cause changes in the temperature of downstream water, and some of the time, but not all of the time, those changes are undesirable. 12 Changes in the temperature of downstream water can be prevented in dams with multiple outlet levels by release of an appropriate mix of epilimnion and hypolimnion water. For some dams without multiple outlet levels, destratifying the reservoir may be feasible. However, the goal of maintaining downstream water temperature, because it requires a mix of warm and cold water, may conflict with the goal of maintaining downstream oxygen levels, which calls for release of warm epilimnion water.
13 Generally, large reservoirs act as sediment traps; the water velocity decreases (compared to the upstream river) and sediment settles to the bottom of the reservoir. Thus, water released from the dam will contain less sediment than upstream water. This is generally viewed as an improvement in water quality. However, the river will tend to restore its equilibrium [sediment] loading by scouring the downstream channel. 14 Also, the reservoir will tend to fill with sediment, which in some cases can require periodic dredging or sluicing. Dredging may temporarily increase sediment load in the reservoir (and hence in the downstream water); sluicing is a deliberate attempt to have the river carry accumulated sediment downstream. 15 14 Sediment release can be reduced by careful dredging or by filtering. There is no evidence in the record to suggest that increased sediment is a major problem. 16
15 When water plunges at high velocity from the reservoir into the downstream river, it becomes mixed with air. Depending on the velocity and turbulence of the falling water and the depth of the receiving basin, this can cause downstream water to become supersaturated--aerated in excess of normal concentration. Supersaturated water does not harm people and is suitable for most uses, but can be fatal to fish; documented fish kills have occurred at a number of dams. 17 16 Supersaturation can be prevented or reduced to non-fatal levels by reducing the turbulence of the falling water (water released from a spillway at the top of a dam is more turbulent than water released through a pipe in the dam), increasing reservoir capacity to reduce the need for spillway releases during flood periods, using a shallow receiving basin below the dam, or constructing spillway deflectors. 18 Supersaturation does not appear at present to be a major problem. The most recent fish kills discussed in the record occurred at Truman Dam in Missouri in 1978 and 1979 while the dam was under construction, and subsequent installation of a spillway deflector has reduced supersaturation to non-fatal levels. 19
17 Dams also cause numerous other changes in the nation's waters, not directly at issue in this litigation. On the positive side, dams can prevent floods, store drinking and irrigation water, provide a clean source of electric power (thus reducing other sources of pollution), moderate stream flow, and provide recreation opportunities. 20 On the negative side, they can indirectly affect ground water quality and reduce stream flow and hence waste-assimilative capacity. 21 In short, dams affect environmental quality in a large number of ways, both good and bad.