Effects of Boatwakes on Streambank Erosion, Kenai River, Alaska

by Joseph M. Dorava and Gayle W. Moore


Abstract

The Kenai River in southcentral Alaska is an economically important salmon river generating as much as $78 million annually in direct benefits. Resource-management agencies are concerned that increased sedimentation and loss of streamside cover associated with accelerated erosion rates caused by boat activity may threaten salmon returns to the river. Bank loss and boat activity were characterized during 1996 along 67 miles of the Kenai River, including a segment of the river several miles long where boat activity is restricted to non-motorized uses. Bank loss in the non-motorized segment of the river was about 75 percent less than that observed in the highest boat-use area of the river and 33 percent less than that observed in the lowest boat-use area of the river.

Dates of peak boat activity coincided closely with chinook salmon returns to the Kenai River and with peaks in measured bank erosion. The boat activity period began in late May, peaked on weekend days in mid-July, and declined in early August. Observed boat traffic on the Kenai River included boats from 10 to 26 feet in length that transported 1 to 8 passengers. The most commonly observed boats were between 16 and 20 feet long and carried 4 or 5 passengers. The number of boats operated by commercial fishing guides represented 40 percent of the boats counted by the Alaska Department of Natural Resources, 55 percent of the boats counted by the Alaska Department of Fish and Game, and 57 percent of those recorded by observers during this study. The maximum boat activity and the maximum bank loss were measured at the RW's Campground study site about 16 river miles upstream from the mouth of the Kenai River. Between July 12 and September 10, 1996, more than 20,100 boats traveled by this site and the streambank along the inside of the meander bend was undercut to a depth of 45 inches at one measuring point. Boat activity and bank loss were greatest in areas of the river between about river miles 9 and 18 and river miles 39 and 46. These two segments of the river are popular residential and fishing areas and have banks composed of non-cohesive soils. In addition, a meandering, un-armored channel makes the banks along these two segments susceptible to erosion.

During 1996, bank loss on the Kenai River occurred primarily during about 60 days in mid-summer when both streamflow and boat activity were at their annual maximums. Streamflow in the Kenai River was generally about 25 to 35 percent below normal during the study period, except for a short period in early August when the rapid release of water stored by a glacier in the headwaters of Snow River increased streamflow above normal rates. Boatwakes contributed about 80 percent of the total energy dissipated against the banks of the study sites during the peak flow and peak boat activity period. At the RW's Campground and the Kenai Keys study sites, water was adjacent to the vegetated riverbanks only for about 60 days during 1996. During this 60-day period, boatwakes accounted for 97 and 94 percent of the energy dissipated against the streambanks at these two sites respectively. At the middle river study site in Soldotna, boatwakes accounted for about 20 percent of the energy dissipated against the banks between June 24 and September 24. Large semi-circular embayments cut into the bank along the inside of meander bends at the RW's Campground and Skilak Lake study sites indicate that the wake-induced erosion may have been prevalent for some time.

Several different types of bank-protection measures were evaluated along the Kenai River for their ability to reduce or eliminate bank erosion. These include complex engineered systems of coconut-fiber biodegradable logs attached to the bank with live willow sprouts and covered with elevated walkways, simple series of spruce trees cut down and cabled to the bank, rock riprap piled against the bank, and vertical wooden retaining walls. With the exception of one site where the cabled spruce trees were washed away during the study and the bank eroded considerably, no significant erosion was visible near the protection systems investigated. These sites include additional ones where cabled spruce trees withstood significant flooding while protecting the bank from erosion.


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Dorava, J.M., and Moore, G.W., 1997, Effects of boatwakes on streambank erosion, Kenai River, Alaska: U.S. Geological Survey Water-Resources Investigations Report 97-4105, 84 p.

(To obtain a copy of this report, call the Earth Science Information Center in Anchorage. In Alaska, call 1-800-USA-MAPS. Outside of Alaska, call 1-907-786-7011.)