These two graphics illustrate why storm flows in Los Angeles’ flood control system (i.e., the Los Angeles River) is more intense when compared the same drop in elevation of the Mississppi River. The area surrounding the length of the Mississippi River (2,000 miles) is primarily open land and allows for more absorbtion and infiltration of stormwater. However, at 1,463 feet, the drop in elevation is HALF of Los Angeles’.
By comparison, the runoff elevation drop in Los Angeles travels from 3,000 feet to sea level (that’s twice the height!) in a fraction of the distance at approximately 51 miles. In addition, because much of the land surface in Los Angeles has been paved in concrete and is mostly non-permeable surface, it does not allow rainwater to be absorbed into the ground and instead, drains off the land carrying surface pollutants into the ocean.