An Evaluation of Stream Restoration Structures in a Coastal Oregon Stream, 1981–1993

A 1.7-km reach of East Fork Lobster Creek, an Oregon coastal tributary of the Alsea River, was treated with mostly full-spanning, rock-filled gabions in 1981 and boulder structures in 1987. East Fork Lobster Creek (EFLC) supports runs of coho salmon Oncorhynchus kisutch and fall chinook salmon O. tshawytscha, winter steelhead O. mykiss, and sea-run and resident cutthroat trout O. clarki. The main objective of treatment was to improve spawning and summer rearing habitat for coho salmon, habitat determined to be lacking during 1980 surveys. Freshets in the winter of 1981–1982 filled all gabion structures with large gravel; the surface area of pool and low-gradient riffle habitats increased but area of high-gradient riffle habitat decreased. From 1985 through 1993, the average number of coho salmon spawners in EFLC increased 2.5 times compared with returns during 1981–1984. In EFLC, treated areas supported significantly more juvenile coho salmon and cutthroat trout and had higher overall salmonid biomass than control areas, whereas age-0 trout (cutthroat trout plus steelhead) and juvenile steelhead showed no increases. For the entire 1.7-km reach receiving treatment, the number of coho salmon juveniles was higher after than before treatment, whereas numbers of steelhead and cutthroat trout fry and juveniles remained constant. Between 1981 and 1992, over 50% of the coho salmon and steelhead spawned on newly deposited, higher-quality gravels associated with 15 gabion structures that fully spanned the bank-full channel width. Quality of gravels impounded by gabions equaled or exceeded the quality of gravels in unmodified areas of the creek. Habitats, primarily pools, created by gabion structures lasted 10 years; however, disintegration of wire mesh tops starting in 1989 caused a slow reduction in pool habitat and gravel riffles at treated sites. All boulder structures remained functional in 1992. Results of this study provide some evidence that interim instream restoration to improve degraded streams and increase salmonid stocks at risk of extinction can be used until long-term watershed restoration strategies have been implemented.