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Stream Channel Changes Associated with Mining and Grazing in the Great Basin

Geological Survey of Denmark and Greenland, Thoravej 8, DK-2400, Copenhagen NV, Denmark;

Ashish Sharma

Utah Water Res. Lab., Utah State Univ., Logan, UT 84322-8200.

^* Corresponding author (rs{at}geus.dk).

ABSTRACT

Characteristics of channel morphology and streambed sediment were sampled at 5- and 10-m intervals, respectively, along a 6.4-km reach of Birch Creek in 1989 and 1992. In this case study we evaluate changes in these channel features using kernel regression analysis. The watershed is located high in the Toiyabe Mountains of central Nevada and has experienced historical grazing and more recent (1986–1989) mining for gold. Exclusion of grazing in the incised lower 1 km of the channel since 1990 did not lead to substantial geomorphic recovery by 1992. The bankfull width did decrease a bit in the grazing exclosure, but baseflow width increased. In both years, bankfull width was about 4 m greater in the exclosure compared with the upstream reaches, reflecting the long-term grazing influences. The shift of grazing pressure into wet riparian areas upstream apparently caused decreases in thalweg depth, increases in fine sediment deposition in the channel, and loss of pool volume in these upstream areas. Some of these changes in substrate sediment could be attributed to inputs from the mine dumps; however, since either fine sediment did not increase or D₅₀ did not decrease in the vicinity of the mines, the changes appear to be more related to recent changes in grazing patterns. Increased fine deposition also occurred in reaches with high volumes of large woody debris. Because of the episodic nature of peakflows that occur in the Great Basin, it may take many years for the full impacts of mining and grazing to be assimilated by the fluvial and riparian systems.