LAUSR

Basin water depth (h) governs the long‐term morphodynamics of river deltas, which are embodied in the grade index (Gindex). The Gindex, a volume‐in‐unit‐time ratio of subaerial sediment allocation to the entire supplied sediment, can be given as a function of the dimensionless basin water depth (h*). Tank experiments reported herein reveal that delta progradation and deltaplain aggradation are suppressed and distributary channel migration and avulsion take place less frequently when the Gindex value is lower (i.e., when the basin water is deeper; h* ≫ 1). If the Gindex~0 (i.e., extremely deep basin water; h*~+∞), the delta can neither prograde nor aggrade, and the distributary channels tend to stabilize. The grade index model helps explain the contrasting morphodynamics of the Liwu Delta (east Taiwan) and Yellow River Delta, as natural examples of deepwater and shallow‐water deltas, respectively.