RECREATIONAL BOAT WAVE IMPACTS ON ST. CROIX RIVER SHORELINES

Scot Johnson 1, Randy Ferrin 2, Wendy Griffin 3, Deborah Konkel 4, and David Pitt 5.
1 Minnesota Department of Natural Resources, 1801 South Oak Street, Lake City, MN 
55041; 2 National Park Service, 401 Hamilton Street, St. Croix Falls, WI 54024; 3 
Washington County Soil and Water Conservation District, 1825 Curve Crest 
Boulevard, Room 101, Stillwater, MN 55082; 4 Wisconsin Department of Natural 
Resources, 1300 West Clairemont, Eau Claire, WI 54201; and 5 University of 
Minnesota, 110 Architecture Building, 89 Church Street SE, Minneapolis, MN 55455.

From the Apple River delta downstream to Stillwater, Minnesota is a reach of the St. 
Croix River that is very popular with recreational power boaters.  Scenic bluffs border 
multiple channels, wooded islands and remote backwaters of this federally designated 
Wild and Scenic River.  Growing concern by river managers and users regarding the 
potential impacts associated with boating activities led to a number of ongoing field 
investigations which began in 1995.  Field investigations included recreational boat 
use and distribution, island geomorphic changes, qualitative shoreline assessments, 
quantitative shoreline surveys, controlled recreational boat wave measurements, 
normal boating activity measurements, wind-generated wave measurements, normal 
and flood flow velocity measurements and shoreline vegetation surveys.  Highlights 
from the investigations include:

Since 1969, small wooded islands downstream of the Apple River have lost area and 
have become more fragmented.  The majority of shorelines classified as experiencing 
medium and high erosion rates were in the main navigation channel.  Boat waves and 
foot-traffic trampling were associated with net shoreline erosion at 9 of the 17 
locations surveyed.  In controlled boat runs, maximum wave heights of 0.4 feet were 
determined to surpass a sediment mobilization threshold.  The sediment mobilization 
threshold was verified by measuring normal boating activity effects.  Turbidity was 
only temporarily elevated by boat wave action because shoreline sediments were 
predominately sand size particles.  Wing-generated waves measured on a windy day 
were much less than the 0.4 feet sediment mobilization threshold and were 
considered only a minor contributing influence to the observed shoreline erosion. 
Shoreline vegetation surveys suggested that boat waves and trampling resulted in 
more bare soil and fewer species of perennial plants.  Recreational boat waves and 
foot-traffic trampling were found to be major contributing influences to the observed 
shoreline erosion.  Overall recreational boating used increased from the 1960s 
through the 1970s and leveled off in the mid 1980s.  Since 1983 boating densities 
have been high enough to trigger river management policy that includes the study 
and/or implementation of surface water use regulations.

Keywords:  recreation, boating, shoreline, erosion, St. Croix River
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