River-dominated deltas are commonly developed at modern bays and lakes and ancient petroliferous basins. Water discharge is an important variable at pay zone scales in river-dominated delta reservoirs, which affects deltaic sand distributions and evolutions. However, it’s unclear how it influences river-dominated delta growth. This paper integrates Delft3D simulations and modern analogs to analyze the effects of water discharge, considering growth time, sediment supply, and coupled effects of sediment properties. High water discharges lead to the formation of lobate deltas, and the water discharge of 1,000 m3/s is a referenced threshold value. Fine-grained, highly-cohesive sediments increase the threshold values of water discharge at which the deltas become lobate from digitate, and vice versa. For the same simulation time, high water discharges favor more rugose shorelines, more distributary channels (especially secondary distributaries), and longer and wider deltas with more land areas. However, for the same sediment supply, high water discharges have few effects on shoreline roughness and the number of distributary channels.