The Froude number is a hydraulic parameter often used to relate aquatic habitats and biotopes to flow intensity. Independently of some trenchant critiques (see, e.g., Clifford et al. 2006), there seems to be no inherent hydrological, geomorphological, or ecological reason that the Froude number (Fr) should be the best indicator of habitat or ecological niches.
Fr is a dimensionless number that describes flow regimes in open channels and is unquestionably useful in many aspects of hydrology, geomorphology, and engineering. It is the ratio of inertial and gravitational forces:
Fr = V/(g d)0.5
Fr < 1 indicates subcritical or tranquil, and Fr > 1 supercritical or rapid flow. But variations in Fr within the subcritical range (where it typically falls) can be significantly related to, e.g., geomorphic units and habitats within channels.
Shawnee Run, Kentucky