Tenn. Code Ann. § 64-3-105

Davidson, Williamson and Rutherford counties, and any municipality or metropolitan government in such counties, are hereby authorized and empowered to contribute to the work of the Mill Creek watershed flood control authority any amount or amounts as recommended by the authority that their respective governing bodies, acting in their sole discretion, shall approve, to be paid either from the general fund of the respective county or municipality or metropolitan government, or from a special ad valorem tax that such legislative bodies are hereby empowered to levy and collect for such purposes, which are hereby declared to be for municipal and county public purposes.