Maintaining fish passage through culverts during flood flows is often impracticable and unnecessary. Extreme low flow periods may also present problems for providing fish passage, and short-term barriers to movement may or may not be important to the survival of species present. It may not be necessary to provide passage at extreme low flows if fish are not attempting to move during this period or if naturally occurring stream conditions limits passage between stream reaches. For any particular species and lifestage the Low Passage Flow (QLP) and High Passage Flow (QHP) define the range of flows to be analyzed by FishXing. These are determined locally or regionally based on knowledge of movement patterns of the species present.

Some states have developed guidelines for determining Fish Passage Flows. For examples of guidelines see the “State and Agency Flow Guidelines for Fish Passage Flows” table in the Flow Guidelines for Fish Passage Flows section.

Low Flow guidelines determine the depth threshold for passage and are based on annual or migration period exceedance percentage from a flow duration curve for the 2-year, 7-day low flow. 

High Flow guidelines determine the velocity threshold for passage and are typically based on annual or migration period exceedance percentage from a flow duration curve.

See also: Flow Guidelines, Reading Flow Duration Curves, Entering Fish Passage Flows