Dams are natural or artificial means of increasing the water storage capacity of running water. Dams, then, are obstacles of some sort located across the axis of streams, creeks, and rivers.
Natural and accidental dams occur when debris accumulates in a waterway. Such an accumulation may occur, for instance when snow or ice pile up and block a river in the spring. Ice dams are notorious associates of spring floods in the northern U.S. and often form when river ice is trapped against bridge abutments, but occasionally these ice dams may occur simply because the large pieces of ice wedge against themselves and the banks and trap more and more of the ice chunks floating down stream. In steep terrain, ice dams can become quite substantial and entrap very large amounts of water. Ice dams are particularly dangerous because they are unstable and release themselves with violent surges of water that may cascade and scour the lower streambed for miles.
Other natural dams occur when mud or landslides migrate downslope until stopped by the bottom of a ravine or valley and then block a stream. Volcanic ash may behave in the same way or the uplift associated with an awakening volcano may also block a watercourse and function as a dam. There is geological evidence in the western U.S. of an enormous natural dam in the headwaters of the Columbia River that blocked a major mountain drainage system impounding a volume of water perhaps equal to the largest of the Great Lakes. 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. The "Scablands" area of Washington are a continuing marker of the scouring of the land which occurred when this "dam" failed.
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. As part of larger natural processes these dams are themselves in process: they are persistent, but transitory and not permanent features of an ecosystem. A specific variation of a natural dam occurs in streams of steep gradient in the accumulation of gross stream load in the repetitive riffle-and-pool pattern familiar to fly fishermen. 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.
Examples of riffle-and-pool dams occur in the stream courses of all the streams that drain from the plateau. 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. Some of the pools created by these natural dams are large enough not only to support fish but have become part of the recreational tradition of the area.
A final kind of natural dam is the beaver dam. Beavers are capable of building dams across virtually any stream or small river. A good survey of many kinds of beaver dams can be found in Lewis H. Morgan's book, The American Beaver. Though dated and not a scientific work, the naturalist's lore and accumulated observations make this a useful guide to beaver construction. Beavers are restocking themselves throughout Middle Tennessee, and the evidence of their work is "piling up" in many of the streams around Sewanee. A particularly good example of beaver dam construction can be observed along I-24 South toward Chattanooga immediately at the Martin Springs Exit. Between the return ramp and the Dixie Highway that parallels I-24, the standing water that can be observed there is the upper end of an excellent beaver dam. This dam was constructed some time in 1994 and utilized materials cut by the beavers as well as brush cut by highway maintenance crews and corn stalks cut from nearby corn fields. In the spring of 1996 this beaver colony had extended stream control downstream by the construction of six additional dams.
Around 1990, a beaver dam was constructed across Battle Creek itself about 100 yards upstream of the log cabin sawmill; this dam blocked the entire stream for most of a year and raised the water level in the pool it created by an additional two feet. Beaver dams can also be noted from time to time along Crow Creek on the way into Sherwood. On Crow Creek, beaver dams serve as a restorative element in overcoming the effects of the channelization of that stream in the 1960's. So domesticated is the expanding beaver population that beaver cuttings and efforts at dam building can be observed on Running Knob Hollow Lake within one block of the Fowler Center on the University of the South campus.
These are the dams build by people as they attempt to use water to meet specific human needs. Man-made dams can be minor affairs such as a couple of logs and a few rocks to create a small pool for livestock or can be the largest human constructions on the earth. Although twentieth century dams along the Colorado and Columbia Rivers in the western U.S. have captured most of our thought about dams, these irrigation and flood control projects are dwarfed by other recent dam construction. The Aswan Nile dam in Egypt, along with recent hydro-electric projects in Brazil and India, the James Bay project in Canada, and the current Three Gorges project on China's Yangtze River are all dam projects of great technical, human, and ecological significance. Any student of waterways or watersheds must understand the impact and significance of these dams. The internet literature on dams and waterways is extensive. A good starting point is the EcoNet Seas and Water Directory particularly the entry, " Dam and Reservoir Impact and Information Archive."
Earthen dams are a common and inexpensive artificial dam that have great utility in both agriculture and in municipal/industrial water supplies. Until the 1950's water supply in the Sewanee area was critically low. The absence of perennial streams on the plateau meant that very little water was accumulated for the use of the growing Sewanee community. During the administration of Vice-Chancellor Edward McCrady and University Forester Charles Cheston, a dam-building program was initiated that erected nearly a dozen dams to contain runoff water and provide a stable water supply for Sewanee. The many lakes visible on the Domain today were created by these dams. In the World War I era on the campus, a dam across the lower portion of Abbo's Alley stream created Hodgson Pond then the only impoundment on the campus. Stone remnants of this dam can be found in the streambed immediately east of the pumping station on Mikell Lane. Further upstream and into the present Abbo's Alley the remnants of a nineteenth century dam/pool constructed as a water supply for a laundry facility can be seen.
While earthen dams are relatively inexpensive and highly serviceable, some earthen dams have failed in recent years creating a national concern about dam safety. In the early 1990's the University acquired a piece of contiguous property that contained a very large lake and dam. This lake was for years know as Day Lake until recently re-named Lake Dimmick. The earthen dam creating this large lake was discovered about fifteen years ago to be flawed on its eastern end because of significant fissures in the underlying rock structure there. State engineers warned that a catastrophic dam failure on Day Lake could result in the inundation of Ladd Cove immediately below in the Battle Creek drainage along I-24 south. Some efforts to repair the dam were made but proved unsuccessful in controlling leakage from the dam. It was estimated that the University of the South faced a $50 million liability exposure in the event of dam failure on this site. In an economically as well as morally interesting corollary to this problem, it was discovered that Tennessee law exempts private dam owners from dam related liability if the property is not used in any way. As a result of this legal finding, the lake and its environs were closed to all users. Students and others may have recently noticed or experienced the careful police patrolling of this site to insure that the lake area is unused.
Any resident or visitor of the Franklin County area must quickly recognize that they are in the center of one of the nation's major hydro-electric and flood control areas. This is the multi-watershed area of the Tennessee Valley Authority. Although it is not so large a river as the Columbia nor so dramatic as the Colorado, the Tennessee River ranks as one of the most completely regulated rivers in the world. TVA operates 29 hydro-electric dams in the Tennessee valley. In the Franklin County area, Tims Ford Dam on the Elk River and Normandy Dam on the Duck River are the important local elements of the TVA system. Note: besides their hydro-electric purpose, these dam sites are open to the public and the cold tailwaters immediately downstream of the dams create good trout fisheries. Visitors to any hydro-electric or flood control dam must be aware of the dangerous water conditions created at such dams. Heed the posted warning signs if you use the water in any way at one of these dams. TVA has several publications to enable people to enjoy dam areas and remain safe in doing so.
Most dam construction today causes controversy. A small-scale dam, but representative controversy, surrounds the construction of the Fiery Gizzard Dam just north of the highway between Monteagle and Tracy City. This dam is currently under construction and has pitted local conservationists led by groups such as SOCM (Save Our Cumberland Mountains) against the local landowners and public utility interests. This site can be easily observed from the small parking lot at the edge of the Fiery Gizzard creek. Also, the masonry elements of an older and much smaller dam can be observed in the streambed near this parking lot.
Small-scale dam building was a common feature of nineteenth century life across America. While a few dams were build for flood control, most of the older dams were built in conjunction with mills. Mill dams can be observed nearby at Falls Mill near Salem on Rt. 64 and at the Ledford Mill near the Nut Cave Trout Pond off the Tullahoma-Shelbyville highway. A small mill dam can also be observed on Boiling Fork Creek about one quarter of a mile upstream of the Rt. 64 bridge near Kurl's Corner in Winchester. An old mill dam can be observed on Bean's Creek just below the confluence of the Falls Mill stream. These dams have lost their original utility and many of them are now damaged or opened, but they recall an important economic and social utilization of water and they serve as an interesting counterpoint of scale to the large hydro-electric projects of the twentieth century.
