Cacti have done away with leaves altogether. Plants such as the ocotillo only produce leaves after sufficient rainfall. Animals also adapt to the arid conditions. Most come out to feed and hunt only at night during cooler temperatures. Because water is scarce at Big Bend, springs and water holes are particularly valuable.
Permanent creeks are rare.
One major exception, the Rio Grande, creates a long, narrow oasis, a ribbon of green, along the southern boundary of the park. The river and springs are vital to the park's wildlife. Like islands in a sea, rugged mountain ranges dot the vast lowland expanses of the Chihuahuan Desert. Some of these mountains, such as the Chisos Mountains at Big Bend, reach high enough elevations to attract additional rainfall. The rain and cooler temperatures allow forests to grow on the upper slopes of these ranges.