The sudden rupture of this dam thousands of years ago and the violent, short-term release of its impoundment created one of the major hydrologic events in the geologic history of North America. The turbulence and standing waves of this event are estimated to have been hundreds of feet high and the soil and rock scoured from the river system in the outbreak altered the mouth of the Columbia River for miles offshore. On a much smaller scale, natural dams can occur in the smallest of streams when the bank caves or when wind blows down a tree across a waterway. These little dams are part of the natural cycle of any stream and are also important for their effect upon surrounding vegetation and wildlife. Natural dams for instance are part of the important ecological cycle of bogs and marshes which create very specialized natural communities in a waterway.
Riffles are the surface manifestation of a minor but nonetheless real underwater dam.
This dam slows down the water, allows oxygenation via turbulent mixing with air, and creates a pool that becomes home to an important and well-differentiated aquatic community. Good examples can be seen along most of the upper portion of Battle Creek, as well as along the stream draining from Lost Cove Cave. A long series of such dams can be observed by wading Savage Creek from the upper waterfall to the valley floor. The pools created behind these natural dams are particularly important in creating sufficient water to sustain fish populations in these small streams. Other natural dams can be observed in Abbo's Alley stream below Morgan's Steep and in the streamway of Barnes Branch below the bridge on Hat Rock Road in Sewanee.