River-mouth process studies and comparisons of river-mouth forms from contrasting environments suggest that sediment dispersal and accumulation patterns are governed by three basic effluent forces and by tide- or wave-induced processes. Neglecting modifications by tides or waves, effluent behavior and consequent depositional patterns depend on the relative dominance of (1) outflow inertia, (2) turbulent bed friction seaward of the mouth, and (3) outflow buoyancy. Inertia-dominated effluents are characterized by fully turbulent jet diffusion, exhibit low lateral spreading angles and progressive lateral and longitudinal deceleration, and produce narrow river-mouth bars. Under most natural circumstances, inertial effects are equaled or exceeded by either turbulent bed friction or effluent buoyancy.
Shallow depths immediately basinward of a river mouth enhance the effects of bed friction, causing more rapid deceleration and lateral expansion. Triangular “middle-ground” bars and frequent channel bifurcation result. Low tidal ranges, fine-grained sediment loads, and deep outlets favor strong density stratification within the lower reaches of the channels. Under such circumstances, effluents are dominated by the effects of buoyancy for at least part of the year. Buoyant effluents produce narrow distributary mouth bars, elongate distributaries with parallel banks, and few bifurcations.
In macrotidal environments where tidal currents are stronger than river flow, bidirectional currents redistribute river sediments, producing sand-filled, funnel-shaped distributaries and causing linear tidal ridges to replace the distributary mouth bar. Powerful waves promote rapid effluent diffusion and decleration and produce constricted or deflected river mouths.
- Geological Society of America