Channel-Unit Hydraulics on a Pool-Riffle Channel

Three riffles, two runs, and a pool were used to examine how water-surface gradient and average velocity varied among channel units over a discharge range of 1.8 to 19.1 m3/s. Alternative hypotheses for water-surface gradient were that water-surface gradient would parallel bed gradient for each unit and remain relatively consistent as discharge changed, or that water-surface gradients would converge among channel units as gradients over riffles decreased and those over runs and pools increased. Alternative hypotheses for average velocity within each channel unit were that differences in average velocity would remain consistent between channel units as discharge increased, or that velocities would converge because average velocity would increase more rapidly in runs and pools than in riffles. These hypotheses were tested on the North Fork Poudre River in Colorado. The North Fork Poudre is incised into granite and has minimal bedload transport under the range of discharges studied, so that channel geometry remained essentially constant throughout the period of study. Water-surface gradient was measured using staff gages at the upstream and downstream end of each channel unit. Average velocity was measured using a salt tracer measured with conductivity probes at the upstream and downstream end of each channel unit. The discharge range during the study includes average annual low flow and peak flow. Water-surface gradient data support neither hypothesis; gradient decreased over all channel units as discharge increased, but the rate of decrease was greater over runs and pools. Velocity data mostly support the hypothesis of consistent rates of increase among channel units with increasing discharge, although one of the three riffles had a much lower rate of velocity increase than the other channel units.