section_path: Geography/Hydrology
section_name: Hydrology
paragraph_text: Hydrology
  
  Nigeria is divided into two main catchment areas - that of Lake Chad and that of the Niger. The Niger catchment area covers about 63% of the country. The main tributary of the Niger is the Benue, whose tributaries extend beyond Cameroon into Cameroon into Chad and the Sharie catchment area. In the Sahel region, rain is less than 500 millimetres (20 in) per year, and the Sahara Desert is encroaching. In the dry northeast corner of the country lies Lake Chad, on a shared water boundary delimitation with Niger, Chad and Cameroon.
  
  The Chad Basin is fed from the north-eastern quarter of Nigeria. The Bauchi Plateau forms the watershed between the Niger/Benue and Komadugu Yobe river systems. The flat plains of north-eastern Nigeria are geographically part of the Chad Basin, where the course of the El Beid River forms the border with Cameroon, from the Mandara Mountains to Lake Chad. The Komadugu Yobe river system gives rise to the internationally important Hadejia-Nguru wetlands and Ox-bow lakes around Lake Nguru in the rainy season. Other rivers of the northeast include the Ngadda and the Yedseram, both of which flow through the Sambisa swamps, thus forming a river system. The river system of the northeast is also a major river system. In addition, Nigeria has numerous coastal rivers.
  
  Over the last million years, Lake Chad in the far north-east of Nigeria has dried up several times for a few thousand years and just as often growing to many times its current size. In recent decades its surface area has been reduced considerably, which may also be due to humans taking water from the inlets to irrigate agricultural land.