3. The impounding capacity of a dam is the total volume of water stored in the reservoir measured at the full supply level. Where bathymetric surveys or other site surveys to measure the impounding capacity with greater precision are unavailable,
(1) the impounding capacity of a dam built across a watercourse is equal to the product of the backflow length multiplied by one-half the reservoir depth and by the average width of the body of water formed by the dam; and
(2) the impounding capacity of other dams is equal to the product of the surface area of the reservoir multiplied by the reservoir depth.
The reservoir depth is the vertical distance between the lowest point of the natural surface of the ground at the downstream toe of the dam and the full supply level.