The Role of River Discharge and Geometric Structure on Diurnal Tidal Dynamics, Alabama, USA

As tides propagate inland, they become distorted by channel geometry and river discharge. Tidal dynamics in fluvial‐marine transitions are commonly observed in high‐energy tidal environments with relatively steady river conditions, leaving the effects of variable river discharge on tides and longitudinal changes poorly understood. To study the effects of variable river discharge on tide‐river interactions, we studied a low‐energy tidal environment where river discharge ranges several orders of magnitude, the diurnal microtidal Tombigbee River‐Mobile Bay fluvial‐marine transition, using water level and velocity observations from 21 stations. Results showed that diurnal tidal attenuation was reduced by the width convergence in seaward reaches and height convergence of the landward backwater reaches, with the channel convergence change location ∼40–50 km inland of the bayhead and seaward of the largest bifurcation. River events amplified tides in seaward regions and attenuated tides in landward regions. This created a region of river‐induced peak amplitude seaward of the flood limit (i.e., bidirectional‐unidirectional current transition), allowing more tidal energy to propagate inland. Tidal currents were attenuated and delayed more by river discharge than water levels, making the phase lag dynamic. The river impacts on the tides were delineated longitudinally and shifted seaward as river discharge increased, ranging up to ∼180 km. Results indicated the longitudinal shifts of river impacts on tides in alluvial systems can be estimated analytically using the ratio of river discharge to tidal discharge and the geometric convergence of the system. Our simple analytical theory provides a pathway for understanding the tide‐river‐geomorphic equilibrium along increasingly dynamic coasts.