If the ancestral Colorado River carried sediment at about the same rate as the present river since the building of Hoover Dam, it may have carried about 3 cubic miles of sediment each century. Now most of the rock debris is being dumped into Lake Powell—the new reservoir behind Glen Canyon Dam. When this, Lake Mead, and other reservoirs ultimately become filled with sediment, the Gulf of California will again be the burial ground. But other events during the Pleistocene also played a role in shaping the area. The Uncompahgre arch was again uplifted and deformed in the Pleistocene soon after the abandonment of Unaweep Canyon. This caused added tilting of the strata and more bending and breaking along some of the folds and faults in the Monument. Capture of East Creek East Creek, which drains the northeastern half of Unaweep Canyon, was forced to change its course during the Pleistocene Epoch by another act of piracy. After capture of the Gunnison River by the newly routed Colorado River, East Creek joined the Gunnison by way of Cactus Park. Then, a tributary of North East Creek headed southward and captured East Creek, as shown in figure 34_D_. Canyon Cutting When the Colorado River was diverted into its new course through the Grand Valley past the Monument, the stream channel seems to have been only about 600 to 800 feet higher than it is today, but the present divide in Unaweep Canyon is now about 2,500 feet higher than the channel. The difference of 1,700 to 1,900 feet was caused by the additional uplift of the Uncompahgre arch during the Pleistocene. Thus, the Grand Valley and its tributary canyons, such as those of Colorado National Monument, were cut since the abandonment of Unaweep Canyon, probably mainly during the Quaternary Period. This suggests that the cutting of the Monument’s canyons began only about 2 million years ago, but that much of the canyon cutting occurred only a few hundred thousand years ago. Indeed, the canyons are still slowly being deepened, lengthened, and widened. As you stand on any of the lookout points and gaze down into the canyons of the Monument, you may well wonder how such immense chasms could have been cut by such puny streams that are dry most of the time. The streams flow only for short periods after heavy thundershowers or after rapid melting of snow. If you are lucky enough to see them flow, you will notice that the water is red or brown because of the suspended mud, silt, and sand. If the flow is large, you may see or hear pebbles and cobbles rolling along the bed. Accordingly, the streams and their cutting tools are slowly deepening the channels. But, you may ask, how does this account for such wide, broad-bottomed, cliff-walled canyons? Such streams act mainly as storm sewers to carry off the rock debris formed by other types of erosion.