”When today a new dam is proposed, river advocates may first feel their rage against the loss of the river section that will be inundated. A reach of a beloved river will be lost—a river no more. But much more that the inundated reach is lost—upstream, downstream, laterally. In the first place, the dam—a discontinuity—creates a break in a line, an obstacle not only to water, but also to plants and animals that evolved to benefit from the river’s original continuity.
Laterally, the destruction of a wide, spreading still water destroys immense riparian habitats. Plan succession is changed in its rate and quality. Terrestrial land migrations across or along the riverway may be blocked. Human use of the valley—what’s left of it — may increase greatly, resulting in overuse. Downstream, the river is a new stream for a long way. All aquatic plants are changed. Water temperatures may be lowed or elevated drastically, completely altering the thermal habitats of fish and invertebrates. Endemic species may be extirpated to extinction…. The food supply to fish and invertebrates in the downstream reach is converted to a resource totally different. Food available to fish then becomes dependent upon the output of zooplankton and sludge worms from the reservoir. Rather than an undimmed community of mayflies and caddisflies. Furthermore, all streambed organisms become subject to unpredictable flows for the dam.
When sediment is deposited in the reservoir, clear water is released through the dam. In a river that used to carry a suspended sediment load, the clear water may be just as much a detriment to the stream as it may seem an improvement to our own eyes. Ironically, clear water has a greater capability to erode, pick up, and carry off new sediment form the river bad and banks (and deposit it farther downstream!)It is called “hungry water”, and the result is an increased erosion of the river’s natural floodplain and riparian zones.
Streambeds are eroded to lower elevations, dropping local water tables. Even upstream, streambed erosion is increased due to upstream migration of the “head cut” (the point where the river flow “falls” into the reservoir) — destroying riffles, pools, and other streambed and bank covers. Biodiversity — upstream, downstream, and alongside — is lost. Quite apart for the drowned streams beneath still waters, dams are losers.”
Source: Wildstream— A Natural History of the Free-Flowing River, Thomas F. Waters, 2000