Simulation of flow and sediment transport in the white sturgeon spawning habitat of the Kootenai River near Bonners Ferry, Idaho

Abstract

Characterization of sediment transport of the Kootenai River in the white sturgeon spawning reach is needed by the Kootenai River White Sturgeon Recovery Team to predict sediment-transport conditions that improve spawning conditions for the white sturgeon (Acipenser transmontanus) in the Kootenai River near Bonners Ferry, Idaho. The decreasing population and spawning failure of the white sturgeon has led to much concern. Few wild juvenile sturgeon are found in the river today. The Kootenai River begins in British Columbia, Canada, and flows through Montana, Idaho, and back into British Columbia. A 15-mile reach of the Kootenai River in Idaho was studied, including the white sturgeon spawning reach that has been designated as a critical habitat near Bonners Ferry, Idaho, and a 1-mile long side channel around the western side of Shorty Island. A one-dimensional sediment-transport model of the study reach was developed, calibrated, and used to simulate the response of the hydraulic and sediment system to varying discharges and water-surface elevations. The model comprises 79 cross sections, most of which came from a previous river survey conducted in 2002-03. Bed-sediment samples collected in 2002 and additional samples collected for this study in 2004 were used in the model. The model was calibrated to discharge and water-surface elevations at two U.S. Geological Survey gaging stations. The model also was calibrated to suspended-sediment discharge at several sites in the study reach. The calibrated model was used to simulate six different management alternatives to assess erosion and deposition under varying hydraulic conditions at the end of 21 days of simulation. Alternative 1 was simulated with a discharge of 6,000 cubic feet per second (ft3/s), alternative 2 with 20,000 ft3/s, alternative 3 with 40,000 ft3/s, and alternatives 4 through 6 with 60,000 ft3/s and represents low to high discharges in the river since the construction of Libby Dam. Sediment deposition was dominant in management alternatives 1 through 4. The streambed in the sandbed reach changed little or not at all. The gravel-cobble reach was more dynamic. In alternatives 1 through 4, deposition was the dominant feature because increasing river discharge alone did not produce boundary shear stresses that can erode and transport streambed sediments. Water-surface slope probably was the limiting factor in these alternatives because backwater conditions flattens the stage throughout the reach. High flows in the river probably would be more effective in eroding the streambed and transporting sediments if water-surface slope was increased. One practical method for increasing the slope is to lower the water level in Kootenay Lake. Two additional alternatives (5 and 6) were simulated to demonstrate the effects of a steeper slope in the study reach. Simulation results from management alternatives 5 and 6 (a discharge of 60,000 ft3/s) were quite different than those from alternatives 1-4. Erosion was the dominant feature in these simulations because water-surface slopes were increased by lowering water levels in Kootenay Lake. Slopes in alternatives 5 and 6 were 2.4 and 3.5 times, respectively, greater than slope in alternative 4. For alternatives 5 and 6, sediment deposition dominated in the gravel-cobble reach while erosion dominated in the sandbed reach. Downstream of Ambush Rock (river mile 151.8) in the sandbed reach, maximum streambed decreased 2 and 3 feet in alternatives 5 and 6, respectively. Decreases also were prevalent in the side channel and averaged 1 foot or greater. White sturgeon eggs have been collected in the study reach since 1994. The largest number of eggs have been collected in the reach adjacent to Shorty Island. Another large number of eggs was located between river miles 149 and 146. Although these reaches for alternatives 5 and 6 were erosional, these reaches are still considered unsuitable spawning habitat because