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August 14, 2003 3:37 PM

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River Types: Tailwater Fisheries

Tailwater Fisheries Gallery


Flaming Gorge, Green River, Utah

A wild river is a rare and beautiful thing; a free roaming spirit that carves its unique signature into the land. During the last 200 years in North America, more than 75,000 dams have been constructed across them. Most were created for flood control, or for reliable sources of water for irrigation and drinking to accommodate growing populations of humans and their livestock, while about a thousand others were built to produce cheap, plentiful electricity. The largest construction project of its time, the dams of the Tennessee Valley Authority were conceived for yet another reason; to reduce mosquito populations that carried the deadly malaria parasite. Lowering and raising water levels weekly in those artificial lakes stranded countless numbers of mosquito eggs and larvae on their shores, where they died. Incidental to that main goal, superb warm water fisheries arose behind each one. Electricity and rapid economic development of the entire watershed were the results of dam building along the Tennessee River. Today, the frigid waters that gush from the bottoms of many TVA dams support significant populations of trout and macro-invertebrates in their tailraces. Where trout did not exist, due to high water temperatures in the unaltered Tennessee River and its tributaries, they now flourish.

Well-known tailwater fisheries include the West and East Branches of the Delaware River in New York State, the White River in Missouri, and the San Juan River in New Mexico. The Madison and Missouri Rivers in Montana, the Green River in Utah, and the Hoositonic River in Connecticut were also remarkably improved as coldwater fisheries after dams were erected across them. Most of these famous rivers are well known because of their high rates of bio-productivity, with the Green River holding the unofficial world’s record. That river supports an astonishing 22,000 trout per mile, as enumerated by electro-shock surveys from the Flaming Gorge dam downstream for some six miles. Other tailwater fisheries have somewhat lower populations of fish, but most tend to harbor significantly more that their free-flowing counter parts.

Madison River, Montana

The reasons for such a high rate of bio-productivity in tailwater ecosystems are many, but the major one is that the fish have a constant food supply, due to a continuous flow of nutrient-rich, cold water throughout the year. The dependability of the trout’s food supply results from the steady input of dissolved minerals and algae from the lake above, which then are either absorbed or fed upon by the abundant, diverse assemblages of downstream filter feeding macro invertebrates. In terms of bio-productivity, many tailwater fisheries compare favorably to limestone streams, and for some of the same reasons. In both there is usually abundant growth of in-stream macrophytes, because of their steady flow rates and the lack of significant flooding. Macrophytes support abundant insect and crustacean communities. Unlike limestone streams, however, the water is usually slightly acidic (typically 6.8- 6.4).

Rivers emanating from the bottom of dams, when managed for the fisheries they have created offer some of the finest and most challenging angling in the world. However, these ecological situations do not always represent a viable solution to the “variableness of nature” problem. When mis-managed as a sport fishery - a situation too often encountered - they are disappointing, even dangerous places to visit. Releasing too little water in summer months leads to fish kills, while excessive cold water releases change placid rivers into raging torrents, too swift and cold to wade safely, and also may occasionally kill fish due to thermal shock. The macro invertebrates also suffer these irregularities, and hatching schedules of well-known species are often quite different from free flowing rivers in adjacent watershed. Very large impoundments with massive dams, such as the ones on the Columbia River in Washington State (over 130), pose even greater potential health threats to migratory species of fish. The high pressure of the water column above forces excess nitrogen into the water as it boils out of the release tubes at the base of the dam. Fish absorb the excess nitrogen through their gills into their blood stream. Moments later, as the fish swim out of these high pressure zones, the partial pressure for nitrogen inside exceeds that on the outside, and the nitrogen inside tries to exit the fish, resulting in bubbles of the gas developing just under the skin of the hapless animal. It is a form of the “bends”, and millions of fish fry die each year, particularly among the Pacific salmons, at the hands of this tragic situation as they attempt to make their way to the ocean.

Expert Fisherman, Green River, Utah

Another more serious drawback to damming a river is that over time, sediment from the watershed above accumulates at the base of the dam. During periods of extreme drawndown sediment is disturbed and enters the river below, bringing with it unwanted agricultural runoff (e.g., salts, pesticides, herbicides), oxygen-depleting silt, and other noxious substances from non-point sources (i.e., roads, houses, service stations, etc., that lie up-stream of the reservoir). All of these contaminants can eventually end up in the river below the dam, and may have significant impact on the life forms of that portion of the river. If one solution to pollution is dilution, then damming a river short-circuits that process.

In the past, controlling rivers for flood control or for the generation of power was the norm. Today, dam building activity has slowed, and in some places has stopped, altogether. The motive is, in some cases, to improve sport fishing, particularly for salmon in our eastern North American coastal rivers. Many dams in Maine, for example, have been de-commissioned by the Federal Energy Regulatory Commission, and now only serve to obstruct the passage of the meager few salmon which each year survive the heavily fished waters of the northern Atlantic Ocean. Slowly, one by one, they are being torn down to permit the “return of the native”.

Are many of the world’s dammed rivers “damned” forever? The future is unclear and will remain so until ancient migratory routes are once again available to anandromous and catandromous fishes.

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