Ecology and management of the South Fork Snake River cottonwood forest

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The report summarizes an investigation of the cottonwood ecosystem along the South Fork Snake River from Palisades Dam to Heise, Idaho. Vegetation dynamics in time and space, with an emphasis on the cottonwood component, was the primary focus. Because riparian vegetation is so intricately related to the river's physical processes, these were studied to the extent that available data and field observation allowed. The South Fork of the Snake River water flow is dominated by snowmelt and its sediment load is strongly influenced by glacial deposits. Its channel form and location on the flood plain is quite stable under most flows. However, the channel can migrate a considerable distance during very high discharges. Due to the inherent stability, sediment budget, and Palisades Dam discharges, the river changed little since dam closure. This lack of change has two main implications: the river should behave now in a similar manner as it did before dam closure under similar discharges, and the lack of high peak discharges due to flood control has inhibited channel migration. The former allows for easier prediction; the latter has affected the cottonwood forest. Floods were the dominant disturbance factor before Palisades Dam closure, and most of the cottonwood forest established on sediment deposited during large floods. Species such as cottonwood and willow that require bare, moist, mineral soil in sunlight for regeneration are declining because of flood control. The smaller floods since Palisades Dam closure have created less area conducive to cottonwood and willow regeneration, and total forest area is shrinking and becoming disproportionately older. For the South Fork, the most important island-forming and cottonwood colonization events are at least 36,000 cfs. Annual floods of at least this size recurred about 8 years apart during the pre-Palisades period. Considering the size of stands created since 1910 and assuming a 300-year longevity for cottonwood stands, a 36,000 cfs event is just enough to create new stands large enough to maintain the cottonwood forest, and this assuming an optimistic cottonwood life span of 300 years. Floods in the 45,000 to 50,000 cfs range are much more apt to maintain the forest because islands formed under this regime are considerably larger than those created under flows about 36,000 cfs. The recurrence interval for 45,000 to 50,000 cfs are 27 and 54 years, respectively. Sediment deposition and water discharge patterns are important for riparian vegetation establishment and survival. Seasonal water levels influence general water availability to plants, while soil characteristics locally influence water availability. Species composition responds to water availability. Dam releases during most of the growing season are similar to the pre-dam condition, and existing vegetation appears to be growing well. However, cattle grazing has impacted vegetation in some places, and recovery will likely be slow in the most heavily-impacted areas.

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Location: COMMUNITY ECOLOGY REPRINT FILE; ELECTRONIC FILE - ECOLOGY

Generic citation: Merigliano, M. F. 1996. Ecology and management of the South Fork Snake River cottonwood forest. BLM Technical Bulletin 96-9. 79 p. plus 10 plates.

SWAP (2/19/2016) citation:
Merigliano MF. 1996. Ecology and management of the South Fork Snake River cottonwood forest. Boise (ID): U.S. Bureau of Land Management, Idaho State Office. 66 p. Tech. Bull. 96-9. [accessed 2015 Dec 3]. http://www.blm.gov/style/medialib/blm/id/publications/technical_bulletin...