example (although not located in North America) is the loss of a Mediterranean fishery due to the construction of the Aswan Dam in Egypt. A nursery ground for species of Mediterranean fish had existed behind a sandbar at the mouth of the Nile that paralleled the coast. The loss of particulate material due to currents in the Mediterranean was compensated for by the addition of new sedimentary material from the Nile. When this renewal ceased, the sandbar eroded and the nursery ground was seriously affected (George, 1972).

Alteration of Flow Patterns

Annual flow patterns have been altered not only by dams, but also by diversions, consumptive uses (irrigation, evaporative cooling), and acceleration of runoff in drainage basins. In urban and suburban areas, rain falls on impervious surfaces and is directed into the nearest watercourse via storm sewers. In agricultural areas, drain tiles, ditches, and channelized streams have the same effect. Water drains much more rapidly from logged areas than from the original forest. The end result of these land uses is that flood peaks are higher, and low stages are lower and longer lasting, than in the past because there is less retention of water in the basin itself (Borman et al., 1969; Karr and Dudley, 1981; Herricks and Osborne, 1985). Changes in the flow pattern often trigger unwanted changes in deposition and erosion. Sediments may accumulate in formerly productive channels and backwaters downstream, or a process of headward erosion can begin. During droughts, there may be too little base flow in these modified streams to support aquatic life and other beneficial downstream uses.

Modifications in floodplains, as well as on the upland drainage, have altered flow patterns. Flood protection levees permit the former floodplain to be used for agriculture, industry, or housing, but it is no longer available for fish and wildlife production, production of hardwood timber, recreation, or the storage and conveyance of floods. It is ironic that these levees actually increase flood heights (Belt, 1975). Sedimentation rates increase on the remaining unleveed floodplains to the point that the native vegetation, including valuable hardwoods, may be smothered.

Boat Traffic

Rasmussen (1983) summarized the stresses created by navigation projects and the boat traffic they support. Stresses associated with navigation dams are similar to those described above. Building canals

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement