Potential Effects of Inundating Salmonid Tributary Habitat due to Increased Impoundment at Howard Hanson Dam

摘要

To augment late-summer flows below Howard Hanson Dam, a 36-foot increase in spring/summer elevation of the Howard Hanson Reservoir is proposed. The increase in reservoir level would seasonally unindate portions of the mainstem Green River and nine other Tributaries in the Howard Hanson Reservoir basin. Upper wahtershed tributaries are annually planted wirh subyearling chinook salmon (Oncorhynchus tshawytscha), coho salmon (O. kisutch), and steelhead (O. mykiss). Adult steelhead planting began in 1992. We conducted periodic fish and habitat surveys in the tributaries in 1992 to: (1) assess the extent and quality of additional tributary habitat impacted by an increase in maximum level summer conservation pool, (2) characterize present fish usage of these habitats, and (3) evaluated the effects of the pool raise on salmonid rearing and potential steelhead spawning. We determined that the proposed pool raise would seasonally inundate approximately 3.2 miles of tributaries with a wetted are of 1,350,00 square feet during typical spring flow, and 808,000 square feet at late-summer flow. This habitat provides short-term rearing for juvenile chinook (most subyearlings appear to emigrate from tributaries by late spring/early summer), and year-round rearing for other salmonids. Potential smolt production from the affected tributary habitat is estimated to be approximately 21,000 to 210,000 chinook salmon, 11,710 coho salmon, and 1,785 steelhead trout About 640,00 square feet of steelhead trout spawning habitat is also avaiable in the reservoir's large tributaries affected by the proposed project. Potenital inundation of steelhead redds during spring refill would probably cuase high embryo mortality.