The Lower Colorado River was originally one of the most erratic and slit-laden rivers in the world. Seasonal discharge and water quality were enormously variable, and the rigorous environment limited the numbers and kinds of aquatic species to a few specialized types. In less than five decades, the river was completely changed by dam construction and formation of large reservoirs for hydroelectric power and irrigation, and by channelization, meanders of the original stream bed that were cut off by dikes became man-made backwaters, which added to the natural backwaters already present along the river. The Lower Colorado River system now consists of three distinct but interacting major habitat types ? reservoirs, inter-reservoir mainstream segments, and natural and man-made back waters. The specialized endemic biota of the Lower Colorado River and its backwaters has undergone much change since man modified the river environment. Endemic fishes are rare or extinct, and entirely new biotic communities have developed in each of the major habitats. In an attempt to increase the recreational potential of the new system, man has introduced many of the fishes he believes are desirable for sport fishing. Currently, the entire sport fishery of the Lower Colorado River system is supported by introduced species.
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