River relocation, or avulsion, is a fundamental process that fills alluvial basins. However, it is difficult to predict avulsion patterns because the landscape conditions associated with different avulsion styles are currently unknown. Two end-member avulsion styles have been documented in modern rivers: progradational avulsions, during which significant floodplain deposition occurs as a new channel is built, and incisional avulsions, during which floodplain erosion captures flow from a parent channel. Here we propose that avulsion style is related to the tendency for overbank flows to erode or deposit sediment (i.e., floodplain morphodynamics). We present a scaling comparison of floodplain erosion and deposition rates, test it with morphodynamic modeling, and show field data from ancient deposits that are consistent with modeling results. These results demonstrate that floodplain morphodynamics may determine how rivers relocate and that assessing the relative influence of floodplain erosion and deposition may be useful for predicting sedimentation patterns in avulsive systems.