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| 'technical' style, Saugatucket R., Rhode Island |
One of the ecological problems with dams is they block the migration of fish upstream to spawn. When the fish in question is a valuable human food source such as salmon, the problem becomes acute. Fish ladders were introduced at least by the 17th century in France according to records. The first fish ladder in the United States was installed in Rhode Island on the Pawtuxet River made from wood. More recent fish ladders are usually constructed from concrete and rebar and their lies the rub. Some fish such as paddlefish are electrically sensitive to reinforcing metal used in the construction that prevents its use by paddlefish. When the City of Providence replaced the wood ladder with reinforced concrete in the twenties, paddlefish numbers in the Pawtuxet collapsed. Fish ladders can be intimidating structures to fish since they are not natural and wreak of man's presence. If the flow over a narrow ladder is too strong it can exhaust already weak spawning fish, preventing the completion of their life journey. Some even require the participation of human volunteers with nets to lift the stranded fish up over the dam
[above photo credit: Rebecca Kessler]. Undoubtably, the haul is an extremely stressful experience for the fish.
The old Kenyon Mill Dam in Richmond, Rhode Island blocked shad and herring and at one time Atlantic salmon since the 1700s. Of a run of thousands only a few hundred survived. The Pawcatuck River salmon run did not make it. A new ecologically sensitive fish ladder was built to aid the remaining fish. Unlike the typical narrow concrete and metal contraption that scare away more fish than those willing to traverse, the new Kenyon Mill fish way mimics a natural stream bed. Boulders sweep out in arcs across the river to create a series of elevated pools which increase in elevation from the natural river bed until they top the five foot dam. Not completely natural looking because of uniformity and concrete retaining walls, the created rapids nevertheless are designed to accommodate the swimming abilities and habits of shad and herring. Despite the considerable effort to create a easier overpass for fish at Kenyon Mill, three other dams downstream from it still block fish trying to enter the river according to the US National Marine Fisheries Services. Populations of 24 North American migratory fish species are at
10% of their historic size. Alewives, blueback herring, shad, trout, smelt, eels, lamprey, sturgeon and other diadromous species have been prevented from reproducing by man's barriers, overfishing, and pollution. In New England alone there are 25,000 dams, many of which are relics or no longer serve a need. About half are privately owned. If America wants fish to dine on fish in the future it is time
to invest in dam removal and stream restoration. Where dams cannot be practically removed because of cost or countervailing values like recreation or power, natural fishways that incite more fish to cross man's obstacle are clearly an answer to the problem of declining migrating fish populations. Natural fishways won't work in every situation, particularly for high dams, but they will work in many places.
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| modern fishway on Neste R., France |
The Kenyon Mill Dam was not removed because the current textile mill owner uses the impounded water for fire suppression. Removal would have cost less than the $925,000 to build the aesthetic fishway. Experts admit that older fish ladder designs were basically "pulled out of a hat" with little regard for the fish's abilities or preferences. The natural approach to fishways originated in Europe in the 1970s. In North America a few fishways were completed by the 1990s. Design is now informed by biomechanics and animal behavior that varies between species. Shad nor herring jump higher than a foot, but herring can navigate turbulence well while shad avoid turbulence. If a fishway is designed naturally, it should allow up to 94% of fish to pass on their way to reproducing. While the Pawcatuck's runs are far from being restored--something needs to be done to the downstream dams--the state is restocking the river with fish and three dams are now fish friendlier. True, fishways are still artificial structures, but the fish seem to like them more.