DYNAMICS OF SEDIMENT TRANSPORT THROUGH A RIVER BIFURCATION

River channel bifurcations are key nodes that distribute water and sediment into two or more channels downstream. Previous studies suggest the dynamics of sediment routing through bifurcations influence bifurcation stability; when unequal amounts of sediment are routed into either channel a stable, asymmetric bifurcation develops. The unequal sediment distribution can arise from either the interaction of different grain sizes with a local bed ramp at one bifurcate arm or upstream channel curvature. Yet, these two ideas have rarely been tested in the field. We present a combined field and numerical modeling study aimed at evaluating the controls on sediment routing through bifurcations. Our field site, the Jefferson River, MT, is a gravel-bed river with a long-lived bifurcation. A bed ramp leads up to the shallower bifurcate channel, which has a width:depth of 11.4, as compared to the deeper channel which has a width:depth of 20. Both channels have a water surface slope of ~0.001. We deployed radio frequency identification (RFID) tagged gravel and cobble clasts upstream of the bifurcation to track their routing during a flood season. We recovered 204 clasts that had been transported through the bifurcation; almost half (46%) were routed into the shallower channel. The factors that most significantly control which clasts were routed into either channel were upstream flow dynamics and grain shape. Surprisingly, grain size had no control on sediment routing, suggesting the bed ramp played a minimal role. Calibrated numerical modeling reveals that clasts generally follow flow lines, resulting in equal routing between the bifurcate arms. Moreover, the shallower arm has smaller Shields Stresses, indicating it will likely alleviate with time. Our results highlight the potential for using RFID-tagged clasts to study bifurcation mechanics and show that, in this case, upstream channel curvature is a more significant control on sediment routing than bed ramp mechanics.