The efficiency of land formation for 10,000 river deltas

River deltas are some of the most economically and ecologically valuable landforms globally, but controls on deltaic land formation are poorly understood. Here we use NASA Landsat imagery of delta land area change combined with fluvial suspended sediment flux data of about 10,000 deltas to investigate land formation efficiencies. We define delta land formation efficiency as the net land area (in m 2) formed per volume of delivered sediment (m 3), a function of deltaic fluvial sediment retention and basin depth. We find that deltas, on average, generate land at 0.1 +/- 0.008 m 2/m 3 of fluvial sediment. Delta morphology exerts significant control on its efficiency. Tide dominated deltas are most efficient at 0.16 m 2/m 3. Wave-dominated deltas produce land at 0.11 m 2/m 3. River-dominated deltas are least efficient and produce land only at 0.05 m 2/m 3. The inefficiency of river-dominated deltas cannot be explained by their sediment loads: tide dominated deltas have greater sediment loads, on average. Instead, river mouth processes including river plume characteristics likely exert an important control on deltaic land formation.