Summary

Three sampling sites were selected at the Kootenai River near Bonners Ferry, Idaho, to represent the varied hydrologic conditions in the white sturgeon critical habitat. Twenty-nine suspended-sediment samples and 25 bedload-sediment samples were collected. Two sets of quality-assurance samples were collected for suspended sediment, and 16 samples were collected for bedload sediment.

Sediment discharge data were analyzed to understand the relations of sediment transport within the white sturgeon critical habitat. The suspended sediment primarily consists of fine material that remains in suspension through the white sturgeon critical habitat. The concentration of sand in the suspended sediment was greatest during flows that coincided with peak tributary runoff. This indicates that most of the sand supply in the system is highly dependent on tributary runoff.

Sediment-transport curves for bedload sediment were not developed because of the limited data available at each sampling site. Trends in the available data, however, reveal vast differences in transport regimes at each site. Bedload-sediment transport in the upper reach of the white sturgeon critical habitat indicates that this reach is a sediment-limited system. The coarse surface sediment probably is caused by the sediment-limited system resulting in an armored channel that prevents the mobilization of the smaller gravels beneath the surface. A great quantity of fine-to-coarse gravel is present in the lower braided reach, but the duration of transport of these gravels is limited by the backwater of Kootenay Lake. When a location is dominated by backwater, transport ceases for bedload sediment larger than sand. Bedload sediment transport in the downstream part of the white sturgeon critical habitat is affected by backwater year round, resulting in fine to coarse-grained sand-dominated system.

Total sediment discharge in the study reach is dominated by fine-grained sediment less than 0.063 millimeters. These fine-grained particles likely remain in suspension through the entire length of the white sturgeon critical habitat. Peak discharge typically occurs during the spring freshet or during the spring sturgeon spawning season flow augmentation pulse (sturgeon pulse). Peak sediment discharge occurred during the spring freshet, when fine-grained particles composed more than 75 percent of the total load. The sturgeon pulse transports fewer fines, but does transport an equal amount of sand compared to the spring freshet. Most of the remaining sediment discharge consisted primarily of fine to medium sands. Coarse sand and gravel discharge represented less than or equal to 3 percent of the total discharge through the white sturgeon critical habitat and coincided with the highest hydraulic gradient. Lack of sediment supply in the upper part of the critical habitat accounts for the lack of bedload sediment transport. In the middle part of the critical habitat near the end of the braided reach, a large quantity of bedload sediment is available for transport, but the backwater effect decreases the hydraulic gradient so much that these surface materials are not transported.