The water balances of four major African wetlands—Senegal, Niger, Sudd and Okavango—are compared through analysis of inflows and outflows, rainfall and evaporation. Annual and seasonal inflows are compared and marked differences are revealed. A simple relation between flooding area and volume is included in the water balance model in order to estimate monthly series of flooded areas over the period of records. These show great differences in the proportions of permanent and seasonal swamp, which help to explain the differences in vegetation distributions.