The snail darter (Percina tanasi) was discovered in 1973 in the lower Little Tennessee River. Prior to construction of impoundments in the Tennessee River drainage, it likely occurred in the mainstem of the Tennessee River and the lower reaches of its major tributaries from Fort Loudon downriver to the confluence of the Paint Rock River in Alabama . Initially thought to occur only in the Little Tennessee River, natural populations of the species were subsequently found at five locations:
- Tennessee River. The darter occupies reaches below the confluence of the Little Tennessee River, below Watts Bar Dam, below Nickajack Dam, and below Chickamauga Dam .
- South Chickamauga Creek. Discovered in 1980 , this population ranges from the Tennessee River confluence upstream 19.5 miles .
- Sewee Creek. A substantial population was discovered in 1991 . In ranges from the Tennessee River confluence upstream 6.0 miles .
- Paint Rock River. Small numbers of darters occur in the Paint Rock River [2, 8].
- Sequatchie River. The snail darter existed in very small numbers in the Sequatchie River, but may be extirpated [2, 8]
It was introduced or colonized seven rivers after being listed:
- Hiwassee River. A population of 710 fish was introduced in 1975 and 1976, growing to an estimated 2,500 fish by 1979 [2, 3]. The population ranges from the 35.8 mile point, upstream to the Bradley/Polk county line and from the Bradley County line to the confluence of the Ocoee River . It was considered relatively abundant in 2004 .
- Holston River. A population of 531 fish was introduced in 1978 and 1979 from the Hiwassee and Little Tennessee Rivers . It ranges from the Tennessee River confluence upstream 14.5 miles . The population grew as habitat conditions improved  and was considered relatively abundant in 2004 . But others report that the population has not done as well as the TVA expected .
- Little River. The Little River was colonized by the Holston River population in about 1979 . It was relatively abundant in 2004 .
- French Broad River. A population was discovered in 1988 upstream from the confluence with the Holston River and is believed to have been colonized by the Holston River population . It ranges from the Tennessee River confluence upstream 30.0 miles . The population was relatively abundant in 2004 .
- Elk River. A population of 425 snail darters from the Little Tennessee River was introduced in 1980 , but was extirpated in the early 1980s .
- Nolichucky River. A population of 61 fish was introduced in 1975, but the program was discontinued after the discovery of sharphead darters in the river for fear of impacting the rare species . It was extirpated in the early 1980s .
- A single snail darter was found in the Ocoee River in 1993 and has not been seen since . It was probably a disperser from the Hiwassee River population .
Assuming that all natural populations existed in 1974 regardless of when they were discovered, the number of snail darter population has increased from six to nine since being placed on the endangered list in 1975. All of the increase is due to the creation of new populations or the recolonization of historic habitat by the created populations.
 USFWS. 2004. Bioloigcal Opinion on the Reservoir Operations Study (ROS) located in the Tennessee River Valley in Alabama, Georgia, Kentucky, Mississippi, North Carolina, Tennessee, and Virginia. U.S. fish and Wildlife Service, Febraury 9, 2004.
 USFWS. 1992. Snail Darter, Percina (Imostoma) tanasi. U.S. Fish and Wildlife Service, Endangered and Threatened Species of the Southeastern United States, www.fws.gov/endangered/i/e/sae15.html
 Fuller, P. 2004. Percina tanasi. U.S. Geological Survey, Nonindigenous Aquatic Species Database, Gainesville, FL. <http://nas.er.usgs.gov/queries/factsheet.asp?SpeciesID=827> Revision Date: 7/27/2004
 Shute, P.W. and D.A. Etnier. 1999. 1999 Report of Region 3 - North-Central. Southeastern Fishes Council , www.flmnh.ufl.edu/fish/organizations/sfc/regionalreports/R3NC1999.htm
 Godwin, J. 1999. Threatened and Endangered Fish of Alabama, http://www.forestry.state.al.us/publication/TF_publications/tfspring99/endangered_fish_of_al.htm
 U.S. Fish and Wildlife Service. 1983. Snail Darter Recovery Plan. U.S. Fish and Wildlife Service, Atlanta, Georgia. 40+pp.
 The University of Tennessee Center for Industrial Services. 1997. Go with the flow: Tennessee’s New Storm Water Multi-Sector Permits. The Tennessee Department of Environment and Conservation and
The University of Tennessee Center for Industrial Services, Nashville, TN.
 Barclay, L. 2005. Personal communication with Lee Barclay, Supervisor, Tennessee Field Office, U.S. Fish and Wildlife Service, October 6, 2005.