The Okavango Delta is produced by seasonal flooding. The Okavango River drains the summer (January-February) rainfall from the Angola highlands and the surge flows ~1,200 km in a month. The waters then spread over the 250 km by 150 km area of the delta over the next four months (March - June). High temperatures cause rapid transpiration and evaporation, resulting in a cycle of rising and falling water levels. The flood peaks between June and August, during Botswana's dry months when the delta swells to three times its regular size.