Palo Duro Canyon is a classic example of a land-form that has been created by the geologic work of _running water_. Undoubtedly the most important single agent of erosion, running water probably does more to wear away the land than all the other geologic agents combined. This is not surprising considering the fact that the earth’s annual precipitation (such as rain and snow) equals about four billion tons of water. Although the amount of precipitation varies greatly from place to place, the average annual precipitation on land is about 40 inches of water. Of this, roughly 25 percent runs off from the land to form streams. When one drives through the park and fords the normally gently flowing waters of the Prairie Dog Town Fork of the Red River he may well wonder if this unimposing stream actually is the geologic agent that is responsible for this deep gorge. But the visitor who happens to be present during a severe rainstorm will soon be convinced, for during heavy rains this gentle stream becomes a raging torrent. As the river increases in size it also becomes a more effective land-shaping tool, for the larger and swifter the stream, the more rock material it can carry. Thus, when flowing at peak capacity, this branch of the Red River becomes a moving ribbon of sandpaper whose load of sand, silt, and gravel has cut and scoured the canyon walls and floor for hundreds of thousands of years. How long has it taken the river to carve this remarkable chasm? Although there is no way of knowing for sure, geologic evidence indicates that the canyon has formed during the last one million years—a relatively short time, geologically speaking. The work of the river is made still more effective by water and sediment which it receives from its tributaries; this added water substantially increases the volume and velocity of the river. Although many of the tributary streams are dry throughout much of the year, they carry large quantities of water during heavy rains. Moreover, because most of these streams flow over rock surfaces which are not protected by thick soil or vegetation, their waters are quickly transported to the master stream. Thus, the volume and velocity of the Prairie Dog Town Fork of the Red River make it possible—especially during flood periods—for the river to carry a large load of rock particles which effectively erodes the stream channel. Where does this rock debris come from? Most of it is eroded from the sides and bottom of the river’s channel.