Riparian vegetation along the middle Snake River, Idaho: Zonation, geographical trends, and historical changes

A baseline study was conducted on an 83-km free-flowing reach of the Snake River between Swan Falls Dam and the Idaho-Oregon border. The research had 2 components: (1) field characterization and inventory of existing riparian flora, vegetation, and environment (soils, topography, streamflow), and (2 ) determination and mapping, using a geographic information system, of hist oric changes in riparian vegetation based on a time series (1938-39, 1957, 1969, 1987) of aerial photographs. The flora was diverse, with 185 species of vascular plants identified, 63 o f which were exotics. Vegetation was structured vertically along the riverb ank gradient into lifeform-defined habitat types: emergent, riparian shrub- forb, tree, transitional grass-shrub, and upland. Riverbank seepage, probab ly of agricultural origin, blurred zonation patterns on some sites and adde d species to the overall flora. Upstream-downstream differences existed in the physical characteristics and vegetation of river subreaches. Coverage of riparian woodland, island riparian and total riparian vegetatio n, and area of islands increased since the 1930s, with the greatest changes in the 1969-1987 interval. Possible contributing factors were (1) signific ant declines in annual minimum flows since the 1950s, (2) decreases in peak flows following completion of Swan Falls Dam and some upstream dams since the 1920s, (3) introduction and spread, of exotic tree species (Elaeagnus a ngustifolia and Tamarix spp.), and (4) possible effects of intensive agricu lture on river sediment load and soil nutrients. The introduction and proli feration of purple loosestrife (Lythrum salicaria) could have considerable future effects on vegetation-channel dynamics in the middle Snake River.