| Abstract/Description or Keywords |
The Kootenay (Kootenai) River of British Columbia and Montana was dammed in 1972 by the Libby Dam that is operated for hydroelectric power and flood control. Subsequently, downstream flows have been stabilized below the prior 1-in-2-year peak discharge. Historical changes in the river valley were assessed using air photos from 1930, 1962/63 and 1992/1994. Prior to damming, the upstream and downstream reaches involve dynamic channels with changing positions and an abundance of barren sand bars. Following damming, the lower Kootenay channel has become fixed in position and barren bars are deficient. Field surveys in 1996 and 1997 involved three sites along each of the upstream and downstream reaches of the Kootenay and two sites along the free-flowing tributary, the Fisher River. Corss-sectional transects were established up the stream banks, and elevation, vegetation, and substrate were surveyed. These confirmed the abundance of barren sand bars along the upper Kootenay versus cobble substrate and a lack of barren bars along the downstream reach. The free-flowing upper Kootenay and Fisher reaches experienced extensive sediment deposition in the riparian zones after the 1996 and 1997 high water (mean changes of 26 and 52 cm for 1997) whereas the lower Kootenay experienced minimal change (<7 cm). Abundant black cottonowood (Populus trichocarpa) recruitment occurred in 1996 and 1997 along the upper Kootenay and Fisher rivers (1997 densities of 536 and 142 seedling/m2) but no seedlings were successfully established along the lower Kootenay. Grasses have encroached to the river's edge along the lower Kootenay, further preventing cottonwood recruitment. Flow stabilization has thus resulted in channel stabilization, minimal cottonwood recruitment, and diminished deciduous shrubs along the lower Kootenay. The conservation of the riparian woodlands along the lower Kootenay would benefit from the reestablishment of more naturally dynamic flow patterns that would include occasional high spring flows. cottonwood seedling; floodplain ecology; fluvial geomorphology; instream flow needs |
