Lake Débo is a lake in the central part of Mali, formed by the seasonal flooding of the Niger River basin. It is in the Inner Niger Delta of the Niger River. During high water stages of the river, the delta formed by lakes, creeks, and backwaters form part of Lake Débo. The inner delta has many wide channels, which are shallow and flooded marshes; this delta extends over a length of 200 miles (320 km) with a width of 50 miles (80 km).
Lake Débo during high flow season, is at a distance of 80 kilometres (50 mi) from Mopti on its upstream, on the southern end and 240 kilometres (150 mi) from Timbuktu at its downstream, on the north-eastern end. It is the largest of many such seasonal wetlands and lakes which form the Inner Niger Delta, and the largest lake within Mali. Its size is largely reduced during the dry season of September to March. The existence of this lake called the "Great Lake" in the inner delta of Niger River between Jenne and Timbuktu in Mali has been established after extensive study of maps of the region extending over a period from 1000 to 1900 AD; 400 maps were studied for the period.