CANYON. In the western part of the United States many rivers have cut deep channels with precipitous sides. Such a gorge is called a canyon, from the Spanish word meaning " tube." Canyons usually occur in the upper course of rivers where the current is swift, and in dry regions where there is not enough rainfall to widen the channel into a broad valley with sloping sides. Some of the most notable examples are the canyons of the Rio Grande, Yellowstone, and Colorado rivers. (See Grand Canyon.)