Testing nodal point relations by tracking sediment through a bifurcation on the Jefferson River, MT, USA

River bifurcations control water and sediment distribution in fluvial systems, yet the physical mechanisms governing sediment partitioning remain poorly validated in natural rivers. We present a comprehensive field test of nodal point relations using radio frequency identification (RFID) tracking of 378 gravel clasts through a meandering river bifurcation in Montana. The study bifurcation features key characteristics for testing existing theory: upstream channel curvature, a 40-cm bed ramp at the shallower bifurcate entrance, and unequal branch geometry. During the 2017 flood season, we recovered 202 (out of 378) transported clasts, which divided nearly equally between arms (100 north, 102 south). Statistical analysis reveals that sediment partitioning is controlled primarily by upstream transverse position ( p < 8×10⁻⁶) and longitudinal deployment distance ( p < 5×10⁻⁵), while grain size, shape, and mass show no significant influence. Calibrated Delft3D modeling combined with theoretical nodal point relations demonstrates that upstream channel curvature effects dominate over local bed-ramp deflection. The nodal point relation that accounts for non-uniform incoming sediment distribution due to curvature best predicts the observed sediment delivery to the northern arm. Our results provide critical field validation showing that curvature-induced sorting begins well upstream of the bifurcation node, bed-ramp deflection plays a secondary role to upstream flow redistribution, and nodal point relations incorporating full curved-flow physics significantly outperform simplified formulations. These findings advance understanding of bifurcation mechanics and provide guidance for improving theoretical models and river restoration designs.