Highlight 8-7 of the HRS Guidance Manual, page 205
For a river, the TDL is 15 miles downstream from the PPE.
For a lake, ocean, or coastal tidal water, direction of flow is not considered. The TDL is drawn as an arc with radius of 15 miles.
If the TDL for a water body includes both a river/stream and an open water body, the sum of the downstream distance and radius of the arc equals 15 miles.