Evaluation of the potential effects of land use on the hydrology and fluvial geomorphology of the North Fork of the Flathead River

Dalby, Charles

The North Fork of the Flathead River forms the northwestern boundary of Glacier National Park. Proposed coal mining and existing timber harvest in the North Fork's international watershed are land uses with potential to alter water-sediment yields and ultimately the morphology and bed-sediment characteristics of the main channel. A baseline for evaluating future channel changes was developed using channel geometry surveys and geomorphic mapping (1:24000). Channel classification and interpretation of historic channel changes show that the North Fork is an alluvial, gravel-bed river with a relatively stable bed and banks. A notable exception is the 16 mile section of channel north of Camas Creek where the channel is unstable and at times braided--a condition resulting from natural sediment contributions from bank erosion. The downstream hydraulic geometry of the channel network was estimated from hydraulic data collected for 11 self-formed, stable channel reaches. Hydraulic similarities between the North Fork and other gravel-bed rivers provide a basis for estimating effects of increased sediment loads on channel morphology. Proposed coal mining, just north of the Canadian border, would produce 2.4 million tons of clean coal annually for 21 years and disturb 8 mi2 of land in the Cabin and Howell Creek watersheds. During mining, if the mine drainage-sediment detention network functions properly, the increase in downstream sediment load and potential for channel changes is small. However, failure of the network could increase sediment supply to the North Fork, ultimately altering downstream channel geometry and bed-sediment characteristics. In response to increased bed-material supply, the channel would be come wider, shallower, and steeper--changes which increase the efficiency of bedload sediment transport. Uncertainties regarding the success of reclamation and performance of the mine drainage network demonstrate the importance for careful monitoring of sediment yield from the minesite. Recommendations for a sediment monitoring program are given.