FLUVIAL ADJUSTMENTS ALONG THE LOWER BIGHORN RIVER, MONTANA FOLLOWING DAM CONSTRUCTION

Within the 16-mile study reach along the Bighorn River from Yellowtail Dam to St. Xavier, MT, local anglers and river technicians have noted for several decades a seemingly progressive abandonment of side channels, which are critical habitat for trout populations downstream of the dam. This abandonment has been observed in the form of decreased boat clearance at both upstream and downstream side channel connections with the main channel. This study evaluates geomorphic changes along the lower Bighorn River associated with the construction of Yellowtail Dam in 1966 and investigates the mechanisms leading to the abandonment of side channels.

Analysis of cross section data from the long-term gaging station in the study reach and from seven sets of rectified historical aerial photography from 1939 to 2006 reveals that vertical incision and lateral migration have been relatively minimal and do not appear to be the cause of side channel loss. Mapping of the active channel, side channels, vegetated islands, unvegetated gravel bars, and overflow channels show that the active channel area has remained similar while the number of vegetated islands decreased dramatically in response to the abandonment of overflow channels following the construction of Yellowtail Dam. The sharp decrease in vegetated islands was followed by a slow increase in their areal coverage from 1970 to 2006. The areal extent of unvegetated gravel bars gradually decreased from 1939 to 2006, and documents the effects of decreased sediment supply and peak discharges in conjunction with vegetation encroachment in response to Yellowtail Dam and potentially the earlier construction of Boysen Dam in 1952. This study proposes that side channels are progressively abandoned by vertical accretion of fine sediment in side channel entrances and mouths; without the scouring effects of larger peak discharges, fine sediment accumulations block lower discharges from entering the side channels at their upstream ends. This work corroborates the results of previous studies on the Bighorn River (e.g., Martin, 1974) and documents fine sediment accumulation and fluvial adjustments through 2006 along the Bighorn River channel.