Although the summer rains fall in Angola in January, they take a whole month to travel the first 1,000 km of the Okavango River, and then they take a further four months to filter through the plants and numerous channels of the final 250 km of the delta. As a result, the flood is at its biggest sometime between June and August, during Botswana's dry winter months. The delta then swells to three times its permanent size, attracting animals from miles around and creating one of Africa's greatest concentrations of wildlife.