Geologically, groundwater is important as an erosional agent. The dissolving action of groundwater slowly removes soluble rod, allowing surface depressions known as sinkholes to form as well as creating subterranean caverns. Groundwater is also an equalizer of streamflow. Much of the water that nows in rivers is not direct runoff from rain and snowmelt. Rather, a large percentage of precipitation soaks in and then moves slowly underground to stream channels. Groundwater is thus a form of storage that sustains streams during periods when rain does not fall. When we observe water flowing in a river during a dry period, it is water from rain that fell at some earlier time and was stored underground