Erosion and sedimentation in the Kenai River, Alaska

Abstract

The Kenai River system is the most important freshwater fishery in Alaska. The flow regime is characterized by high summer flow of glacial meltwater and periodic flooding caused by sudden releases of glacier-dammed lakes in the headwaters. Throughout most of its 50-mile course across the Kenai Peninsula Lowlands to Cook Inlet, the river meanders within coarse bed material with a median diameter typically in the range of 40-60 millimeters. Every nontidal section of the stream is a known or potential salmon-spawning site. The stream is underfit, a condition attributed to regional glacial recession and hypothesized drainage changes, and locally is entrenched in response to geologically recent changes in base level. The coarseness of the bed material is explained by these characteristics, combined with the reservoir-like effects of two large moraine impounded lakes, Kenai and Skilak Lakes, that formed as lowland glaciers receded. Throughout the central section of the river the channel is effectively armored, a condition that may have important long-term implications for the ability of this section of channel to support the spawning and rearing of salmon. The 3.8 river miles of channel below Skilak Lake contain submersed, crescentic gravel dunes with lengths in excess of 500 ft and heights of more than 15 ft. Such bed forms are highly unusual in streams with coarse bed material. The dunes were entirely stable from 1950 to at least 1977, so much so that small details of shape were unmodified by a major glacial-outburst flood in 1974. The features are the product of a flood greatly in excess of any recorded discharge. The entrenched section of the channel has been stable since at least 1950-195, only negligible amounts of bank erosion are indicated by sequential aerial photos. Bank erosion is active both upstream and downstream from the entrenched channel, however, and erosion rates in those reaches are locally comparable to rates in streams of similar size. Although erosion rates have been generally constant since 1950-1951, evidence suggests a possible recent decrease in bank stability and an increase in erosion that could be related to changes in river use. The high sustained flow of summer encourages a variety of recreation-related modifications to the bank and flood plain--canals, groins, boat ramps, slips, embankments, as well as commercial developments. As population and recreation use increase, development can pose a hazard to the productivity of the stream through increased suspended-sediment concentration resulting directly from construction and, with greater potential for long-term impact, indirectly from bank erosion. A short-term hazard, to both stream and developments is the cutoff of meander loops, the risk of which is increased by canals and boat slips cut in the surface layer of cohesive, erosion-resistant sediment on the flood plain within nonentrenched meander loops. A significant long-term hazard is an increase in bank erosion rates resulting from the loss of stabilizing vegetation on the high (up to 70 ft) cutbanks of entrenched and partially entrenched sections of channel. Potential root causes of erosion vegetation loss are river-use practices, meander cutoffs, and groin construction.