Most of us agree that an uninterrupted wild river is a rare and beautiful thing: a free roaming spirit that is entitled, by its very nature, to have its way with the countryside. Despite this popularly held view, during the last 200 years in North America, alone, over 80,000 dams have been constructed across them (Figure 36). 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 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 10 of the 29 TVA dams support significant populations of trout and macroinvertebrates in their tailraces (Figure 37). Where trout did not exist, due to high water temperatures in the unaltered Tennessee River and its tributaries, they now flourish.
Other 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 Housatonic 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 on downstream for some six miles. After that, the numbers “plummet” to a mere 8,000 per mile. This number is still well above the average trout density for unimpeded trout streams.
The reasons for such a high rate of bio-productivity in tailwater ecosystems are many, but the major one is that the fish enjoy 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 become absorbed and fed upon by the abundant, diverse assemblages of downstream filter feeding macroinvertebrates. In terms of bio-productivity, many tailwater fisheries compare favorably to limestone streams, and for some of the same reasons. In both situations there is usually abundant growth of in-stream macrophytes due to the steady flow rates and the lack of significant flooding. Macrophytes support abundant insect and crustacean communities, and provide adequate cover for trout, making them hard to prey upon. Unlike limestone streams, most tailwaters are slightly acidic (typically 6.4- 6.8).
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, 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 macroinvertebrates also suffer these irregularities, and hatching schedules of well-known species of mayflies in particular are often quite different from free-flowing rivers in the same watershed. Very large impoundments with massive dams, such as the ones on the Columbia River in Washington State, pose even greater potential threats to fish. The high pressure of the water column above the dam forces excess nitrogen from the atmosphere (76% of our atmosphere is composed of 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 these hapless animals. It is a form of the “bends,” and millions of fish fry (mostly salmon) die each year because of this as they attempt to make their way to the ocean.
Another more serious drawback to damming a river is that over time, sediment from the watershed above accumulates at the base of the dam on the reservoir side. During periods of extreme draw down, as routinely happens almost every fall, for example on the West Branch of the Delaware River, sediment is disturbed and enters the river below, bringing with it unwanted agricultural run-off (e.g., salts, pesticides, herbicides), oxygen-depleting silt, and other noxious substances from non-point source run-off (i.e., roads, houses, and service stations that lie upstream of the reservoir). All of these contaminants can eventually end up in the river below the dam, and have significant negative impacts on the life forms of that portion of the river.
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. Removing most of them to improve sport fishing, particularly for salmon in our eastern North American coastal rivers, is now in full swing. Slowly, one by one, they are being torn down to permit the “return of the native”. The American Rivers Organization (www.americanrivers.org) provides a useful list of dams that were removed from 2010 to 2015. It documents the demolition of 310 dams in over 21 states that no longer prevent fish from living out their lives in peace.
What about the rest of the world’s dammed rivers? The future is unclear and will remain so until ancient migratory routes are once again available to anadromous (e.g., striped bass, salmon, shad) and catadromous fishes (e.g., eels). From a human perspective, however, it is still the mystery and wonderment of flowing water itself, altered or not, that pulls us to it. This is the world some of us instinctively seek from time to time in rituals of spiritual self-renewal. Their joys and freedoms are ours to enjoy, as well.