What follows are the minimum criteria used to define these protection zones; individual protection areas may vary somewhat, based on individual circumstances or a desire by the local water planners to protect a larger area than the minimum.

• For most intakes that draw from reservoirs, the water supply protection area will include the entire watershed, segmented (as size permits) into subsections according to the following scheme.
• For intakes in extremely large reservoirs (e.g. Grayson Lake), the water supply protection area includes three protection zones.  These are intended to parallel the protection areas used in the Wellhead Protection Program.  If the intake is in a river embayment or the upper reaches of the reservoir, segmentation rules for river intakes may apply.

Zone I   -   The Critical Zone begins ¼ mile below the intake site and extends 1 mile upstream of the intake, measured in the thalweg (channel), and on the land along each side, ¼ mile from normal pool level of the reservoir (or to the nearest watershed boundary if it is within ¼ mile), and up  3rd order streams.  If this segment of the reservoir is more than ½ mile wide and the thalweg is on the intake side of the reservoir throughout this segment, the Critical Zone may include only the intake shore of the reservoir.

Zone II  -   The Zone of Responsibility extends the protection area to 5 miles above the intake and to ½ mile on both sides of the reservoir (or to the nearest watershed boundary if it is within ½ mile).  The protection area also extends into any tributaries that are 3rd order of above (on 1:24,000 scale maps), for a distance of 5 miles above the intake.

Zone III  -  The Zone of Potential Impact extends the protection area to 10 miles above the intake and includes the area of any 14-digit hydrologic unit that is adjacent to any stream that is 3rd order or above.