Evapotranspiration in the Okavango swamps exceeds rainfall by a factor of three, yet saline surface water is rare. Brines develop in the ground water beneath islands, however, and very strong lateral concentration gradients develop. These arise as a result of transpiration of trees which grow around the fringes of islands as well as by capillary evaporation of ground water from the interior of islands. Precipitation of calcite and amorphous silica from the ground water occurs beneath island fringes.