The Niger's tributaries – the Béli, Gorouol, Goudébo, and Dargol – are seasonal streams and flow for only four to six months a year. They still can flood and overflow, however. The country also contains numerous lakes – the principal ones are Tingrela, Bam, and Dem. The country contains large ponds, as well, such as Oursi, Béli, Yomboli, and Markoye. Water shortages are often a problem, especially in the north of the country.