FishXing uses a flow dependent ratio to estimate the minimum plunge pool depth at the crossing outlet needed to support fish leaping.  Sufficient pool depth for leaping requires the pool depth be equal to or greater than the drop height:

where:

D = Depth of the plunge pool

H = Water surface drop at the outlet, measured from the water surface at the culvert outlet to the water surface of the plunge pool.  

If the pool depth (D) is less than the drop height (H), FishXing reports a “pool” barrier at that particular flow. The rationale for this ratio is discussed in the Leaping from Plunge Pools section.