Lake Chad is a variable body of water. Its measured surface area typically fluctuates seasonally, peaking in late October or early November, then shrinking by more than half by late April or early May. The lake is dotted with numerous islands, which can coalesce into larger land areas during periods of extremely low surface levels. The volume of the lake reflects local precipitation and the discharge of its catchment area, balanced against losses through evaporation, transpiration, and seepage. The lake is fed chiefly by the Chari (Shari)-Logone river system, which accounts for about four-fifths of the inflow. Of the remaining inflow, most is contributed by the Ebeji (El-Béid) and Yedseram rivers. Losses to evaporation and the transpiration of aquatic plants amount to approximately 100 inches (2,500 mm) each year. It is probable that up to another 10 inches (250 mm) replenish groundwater supplies in the adjacent Manga and Kanem lowlands and pass as underflow through the El-Ghazal.