SIDE CHANNEL ABANDONMENT ALONG THE LOWER BIGHORN RIVER, MONTANA

Within the 16-mile study reach along the Bighorn River from Yellowtail Dam to St. Xavier, MT, local anglers and river technicians have noted a seemingly progressive abandonment of side channels for several decades. 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 and investigates the mechanisms leading to the loss of side channel connectivity at frequent discharges.

Analysis of historical cross section data and rectified historical aerial photography reveals that vertical incision and lateral migration do not appear to be the cause of side channel abandonment. Instead, side channels are progressively abandoned by vertical accretion of sediment in side channel entrances and mouths; without the scouring effects of larger peak discharges, sediment accumulations block lower discharges from entering the side channels at their upstream ends and allow for the establishment of vegetation at the entrances.

Sediment transport was evaluated with a two dimensional (2D) hydraulic model in a static condition, whereby the bed did not undergo deformation and discharges were steady. Conditions of sediment transport were evaluated across the spectrum of bed motion, from zero transport to vigorous transport. The numerical modeling of sediment motion was coupled with observation of sediment transport using particle tracers, resulting in a validation of sediment results provided by the model. Bed material data were collected in October 2011 to populate the numerical model with representative sediment size distributions.

Repeat surveys of side channels before and after recent high flow releases show variable lowering of bed elevations and channel widening; however, releases from Yellowtail Dam alone are not likely to provide conditions to reverse the trend of side channel aggradation. At best, planned high flow releases will stop the aggradational trend. Without the supply of upstream sediment and unregulated flows, the fluvial processes that drive side channel adjustments are significantly modified to where the system has been altered from one of dynamic adjustment to one of lateral stability.