A landslide once dammed the Virgin River forming a lake. Sediments settled out of the quiet waters, covering the lake bottom. When the river breached the dam and the lake drained, it left behind a flat-bottomed valley. This change in the character of the canyon can be seen from the scenic drive south of the Zion Lodge near the Sentinel Slide . This slide was active again in 1995, damaging the road.
Flash floods occur when sudden thunderstorms dump water on exposed rock.
With little soil to absorb the rain, water runs downhill, gathering volume as it goes. These floods often occur without warning and can increase water flow by over 100 times. In 1998 a flash flood increased the volume of the Virgin River from 200 cubic feet per second to 4,500 cubic feet per second, again damaging the scenic drive at the Sentinel Slide. When geologist Clarence E. Dutton wrote that description in 1880, southern Utah was a wild, rugged country of little-known canyons and plateaus. Slowly, scientific reports, magazine articles, and photographs spread the word that deep within this remote territory lay the scenic phenomenon of Zion.