All oasis in deserts are formed by water. In most deserts, you indeed can find water, as already stated, usually coming from underground sources. Deserts in their natural form are not devoid of vegetation. Some dune fields and salt flats are, but even the Sahara only is 14% sand-covered. Most is covered in so-called desert scrub, or is semidesertic, specially towards the Sahel. So there is always seeds. If water is present, an otherwise small, scrawny shrub can grow into a sizeable tree and other plants find suitable conditions. In North America’s deserts, this frequently can be seen on mezquite. In the open desert they barely form a clump of spiny twigs and a huge root, near water they will grow to more than 20 ft in height. Usually, such oasis can be seen as green spots from dozens of miles away and attract people. In the Sahara and Arabian Deserts, these people would then plant date-palm offshoots there. Date palms grown from seeds are half males, and the females usually give low quality fruit. So you plant high-quality offshoots. Once established, they will multiply as long as they can reach the water.

The largest oasis in deserts are created by rivers. In Egypt, there is the largest one in the world. One can irrigate, and the water table along the Nile is close to the surface. Similar oasis are formed along the Indus in Pakistan and Euphrates in Syria and Irak. Even along the Rio Grande you find a long, narrow oasis. In the U.S. Southwest, this is called riparian vegetation or in Spanish: “bosques”, meaning woodlands. In New Mexico, you can see a ribbon of green stretching from Albuquerque towards El Paso, separating vast expanses of dry desert scrub.

A nice example of a natural oasis can be found in Organ Pipe Cactus National Monument, Arizona, named Quitobaquito Springs. A lush grove of trees and grasses with all sorts of animals. This is the last water until the Colorado River, some 120 miles to the West, allegedly the longest stretch without water in the North American Desert. In Death Valley, one is never more than 20 miles away from drinkable water.