A method for objectively identifying pool, run, and riffle habitats from physical measurements

Abstract

Visual assessments of habitat type (pool, run, riffle) and physical measurements at 1112 points in a gravel‐bed river were used to derive an objective method for distinguishing pool, run, and riffle habitats. Water depth, velocity, and substrate composition were measured at each point and were used to calculate average substrate size, relative roughness, velocity/ depth ratio, and Froude number. Water surface slope was measured between each of 59 cross‐sections. The velocity/depth ratio, Froude number, and slope were the best determinants of habitat type. Velocity/ depth ratio and Froude number showed the most significant differences between habitat types. In the study river, pool habitat was associated with velocity/ depth ratios of less than 1.24 and Froude numbers less than 0.18, riffles with velocity/depth ratios of more than 3.20 and Froude numbers in excess of 0.41, and run habitat with intermediate values. Riffle habitats were characterised by slopes greater than 0.0099 and run and pool habitats by lesser slopes. Simple classification rules, based on water surface slope and either velocity/depth ratio or Froude number, correctly classified 65–66% of the habitats. Froude number and velocity/depth ratio are suggested as easy‐to‐measure indices which will provide an objective method of classifying habitat type and provide a base for between‐river comparisons.

Keywords:

• pool
• run
• riffle
• habitat classification
• Froude number
• habitat type
• river hydraulics