The length of time between events of a given magnitude. Anderson and Calver (2004) TIBG 29, 1 note that ‘landscape form depends on the overall effect of past frequency distributions of sizes of geomorphic events, and on the weather conditions’ return’. In hydrology, the return period has an inverse relationship with the probability that the event will be exceeded in any one year. See Keylock (2005) Adv. Water Resources 28, 8 on flood recurrence intervals. See also Srikanthan and McMahon (1985) Hydrol. Scis 30, 2 on calculating the recurrence interval of drought events through the stochastic analysis of rainfall and streamflow data.