Without annual flooding, however, erosion rates outpace deposition, causing a slow decline in sand bars and terrace formations. Four major channel types occur in the Grand Canyon. Easily-eroded shales give way to wide valleys where the river meanders freely, creating cobble bars and a wide, shallow flow. In areas of resistant limestone and sandstones, intermediate valleys restrict meanders and a pattern of pools and rapids is common, depending upon the frequency of tributary fans. Fractured igneous and metamorphic rocks constrict the river further, in narrow channels often bordered by uneven cliffs and slopes.
Deep pools form in fracture zones and again, rapids form at tributary canyons.
The final channel type is found along a twenty-five mile stretch where the Colorado cuts through the massive Muav limestone formation. Uniform, vertical cliffs transform the river into a sluiceway with little variation in flow. I recommend it, combined with the Kieffer piece below, as a great introduction for fluvial neophytes like myself. Kieffer, Susan Werner. Hydraulics and Geomorphology of the Colorado River in the Grand Canyon.