An oasis is a fertile area in the midst of a desert. The water source is generally a spring; though in the Sahara many oases have sprung up around wells. An oasis may occur at the point where a river flowing from a wetter region crosses the desert on its way to the sea, such as the Nile in North Africa or the Indus in South Asia.
Wadis are intermittent streams that flow only after heavy rainfall but they often have a hidden flow under their beds which can reach the surface to create oases. Aquifers with recharge areas outside the desert can "pipe" water to oases under long stretches of arid desert. These recharge areas are usually moutains suitably sited for catching rain. The natral flow can be increased by pumping the groundwater but if the rate of pumping exceeds the water flow into the aquifer in the recharge area the wells will dry up.
Oases may be hundreds of square kilometers in area, or merely a few trees clumped together. Most Saharan oases contain large numbers of date palms, and there may also be many fields of cereals, vegetables, and fruits. There is often a settlement at the oasis, sometimes with as many as 30,000 inhabitants.