The average annual precipitation in Jefferson County is about 53 inches (1,346.2 millimeters) or about 2,820 mgd (million gallons per day), which is equivalent to 124 m³/s (cubic meters per second). Part of the rainfall (about 1,130 mgd or 50 m³/s) runs off directly into streams, and the remaining 1,690 mgd (74 m³/s) replenishes soil moisture to underground reservoirs.

Potential sources of ground water in the county are limestone, dolomite, sandstone, and chert aquifers. Wells developed in these aquifers may produce as much as 0.5 mgd (0.022 m³/s) per well. In some areas, wells developed in the limestone, dolomite, and chert aquifers may produce more than 0.5 mgd (0.022 m³/ s) per well.

Potential sources of surface water in Jefferson County are the Black Warrior and Cahaba Rivers, Locust Fork, and Valley, Village, Fivemile, Shades, and Turkey Creeks. Average flows at the mouth of these streams or at the point where the stream leaves the county are 4, 070, 250, 1, 230, 360, 100, 120, 100, and 90 mg (178, 11, 54, 16, 4.4, 5. 3, 4. 4, and 3.9 m³/s), respectively. Low flows (7-day Q₂) are generally in the range of 10 to 100 mgd (0. 44 to 4.4 m³/s). Most streams, however, are affected in some way by urbanization. These effects must be considered and better defined before extensive use of streamflow is made.

Water from the limestone and dolomite aquifers generally is moderately hard to very hard, contains less than 0.3 mg/1 (milligrams per liter) iron, and has 3 median value of 150 mg/1 dissolved solids. Water from the sandstone aquifer generally is soft to moderately hard, contains iron in excess of 0.3 mg/1, and has a median value of 210 mg/1 dissolved solids.

The concentrations of mineral constituents of surface water generally become progressively greater in the downstream direction. The increase in concentration of mineral constituents is attributed to inflow of municipal and industrial waste in the downstream reaches of the stream. This waste disposal has degraded the quality of water in Shades, Valley, Village, and Fivemile Creeks, and Locust Fork.

The average water use in Jefferson County was estimated to be about 200 mgd (8.8 m³/s) in 1970. The major sources of this supply are Lewis Smith Lake, Inland Lake, and Lake Purdy. These three sources supply water for about 75 percent of Jefferson County's population or 66 percent of the water used.