The modification of a flood wave due to bank storage effects can be calculated by using numerical methods. The dynamic equations describing one‐dimensional open channel flow and the equation for two‐dimensional transient groundwater flow are solved simultaneously, coupled by an expression for flow through the wetted perimeter of the channel. Numerical experiments indicate that flood waves may be modified considerably by bank storage, particularly in the lower segments of a long reach, and that the degree of modification is influenced markedly by the hydraulic conductivity of the aquifer.