Settlement in Botswana has largely been determined by the availability of water. Until the recent introduction of boreholes, this has meant that human habitation has been limited to places with reliable surface water. In the north-west, people are concentrated along bodies of permanent water, such as the Okavango and Chobe rivers. Full utilisation of the rivers and their resources has in the past been restricted by the presence of disease vectors such as tsetse fly and mosquitoes.