Jurassic and Cretaceous volcanics often form impermeable units, containing aquifers, along with the Chouf, Abeih and Chekka Formations. The Chekka submarine springs are an example of a confined Cennomanian and Turonian aquifer system. Artesian springs are common along the north coast. Underground springs and melting snow feed 11 rivers in Mount Lebanon and two in the Anti-Lebanon mountains. Annually, Lebanon receives 80% of precipitation between November and March, a total of of precipitation, 30% of which falls in the Bekaa. According to the UN, Lebanon's water demand already exceeds supply.