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.)