Threats

The Effects of Motorized Watercraft on Aquatic Ecosystems Timothy R. Asplund, Wisconsin Department of Natural Resources, Bureau of Integrated Science Services and University of Wisconsin, Water Chemistry Program March 17, 2000 [--Excerpted--]

Why is shoreline erosion important in aquatic ecosystems? Shoreline erosion may affect water clarity in near shore areas, shading submerged aquatic plants as well as providing nutrients for algal growth. It can interfere with fish use of shallow water haitat, as well as wildlife use of the land-water edge. Excessive shoreline erosion can negatively affect property values and can be expensive for riparian dwellers to prevent and control.

How might boats affect shoreline erosion? Boats produce a wake, which may in turn create waves that propagate outward until dissipated at the shoreline. Wave height and other wave characteristics vary with speed, type of watercraft, size of engine, hull displacement, and dis tance from shore. Propeller turbulence from boats operating in near shore areas may also erode shorelines by destabilizing the bottom.

What do we know? Waves or wake produced by boats is the primary factor by which boats can influence shoreline erosion. Wave heights depend upon speed, size and draft of boat, but can reach heights of 40-50 cm (15-20 in.) equivalent to storm-induced waves. However, wave heights dissipate rapidly as they move away from the boat, while wind waves increase with larger distances. Therefore, river systems, channels connecting lakes, and small lakes are likely to be most influenced by boat-induced waves, as boats may operate relatively close to shore and wind-induced waves are reduced. Shoreline erosion has been documented in river systems and has been attributed to frequency and proximity of boat traffic. Loosely consolidated, steep, unvegetated banks are more susceptible to shoreline erosion.

What can we do about it? No-wake zones are designed to minimize boat wake, so the obvious solution would be to use no-wake zones to limit shoreline erosion, particularly in channels or small sheltered lakes (i.e. areas where effective wind fetch is less than 1000 feet). Currently in WI, boats are restricted from operating at speeds greater than no-wake within 100 feet from fixed structures such as boat docks and swimming platforms. Many lake communities have established no-wake ordinances at 100 feet from shore or more. Seawalls and riprap have been used extensively in lakes and rivers to prevent shoreline erosion; however, these engineering approaches have little wildlife value and are expensive. Maintaining and restoring natural shorelines would help reduce the impacts of all types of waves on shoreline erosion.

Management of reed growth, control, and removal poses a challenge for shoreline homeowners and waterways volunteers. Care must be exercised when reeds are removed to keep erosion and bank sloughing from filling the channels.

Approved bulkhead placement, with appropriate landscaping, may prevent waterway narrowing and erosion which leads to shallow channels.

Debris such as logs, old dock material, storm damaged tress and limbs, etc., are unforeseen obstacles until they damage a watercraft. Locating, tagging, photographing, and arranging for removal is required.

Tree branches, or trees leaning over the waterways that threaten to damage boats and passengers should be pruned, or removed.