and locks around natural barriers sometimes leads to unwanted introduction of species (the classic example is the invasion of the Great Lakes by the sea lamprey after Niagara Falls was bypassed by the St. Lawrence Seaway). Wing dams are dikes, perpendicular to the shore, that confine the main flow, thus creating a stable channel that tends to maintain adequate depht for navigation by scouring. Sedimentation occurs between the dikes, filling in the productive channel borders. Closing dams across side channels have the same engineering function and the same side effects as wing dams.

Water displacement, propeller wash, and wakes from boats resuspend bottom sediments, increase bank erosion, and can disorient or injure sensitive aquatic species. Aquatic organisms may also be struck by hulls or propellers. Finally, waste discharges and accidental spills from boats or loading facilities can introduce pollutants and exotic species.

Acid

Conducted between 1984 and 1986, the national surface water survey (NSWS) was one of the first activities undertaken by the National Acid Precipitation Assessment Program (NAPAP, 1990).

From a target population of 59,000 stream reaches (211,000 km), overall only 8 percent, or 4,520 reaches (7,900 km), were found to be acidic. Proportions ranged from less than 1 percent in the western United States and southeastern highlands to 39 percent in Florida, but in all other NSWS regions, 12 percent of fewer of the streams were acidic. The major causes of acid streams are acid deposition, acid mine drainage (Box 5.4), and naturally occurring organic acids.

In 47 percent of the chronically acid streams, the dominant acid anions derived from deposition (via acid rain, acid snow, acid fog). In the majority of these streams, sulfate concentrations exceeded base cation concentrations, indicating that the acidic conditions were caused by sulfuric acid. The most likely explanation for the loss of brook trout populations in the Adirondacks is recent acidification caused by high inputs of atmospheric sulfate (NAPAP, 1990).

In 27 percent of the acid streams, organic acids are the main source of acid ions. These streams are located in Florida and the Mid-Atlantic Coastal Plain and are associated with wetlands or organic soils. In Florida, healthy largemouth bass populations are found in waters with pH ranging from 4.0 to 4.5, and there do not appear to be significant population losses. In contrast, the Mid-Atlantic Coastal Plain has experienced a continuing decline of anadromous species since the 1970s. Some streams whose acidity was formerly caused by organic



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