Effective discharge in Rocky Mountain headwater streams

- Two power functions describe flow frequency distributions in snowmelt regimes.
- Scaling relations indicate gravel rating curve steepness in headwater streams.
- The sum of the exponents of flow frequency and bedload rating curves indicate Qeff.
- The steepness of bedload rating curves and stream type mainly affect Qeff.
- Qeff is near Qmax in plane-bed streams and near Qbf in some step-pool streams.

SummaryWhereas effective discharge (Qeff) in mountain streams is commonly associated with a moderate flow such as bankfull discharge (Qbf), this study found that the maximum discharge (Qmax), and not bankfull discharge, is the channel forming or effective flow for gravel transport in plane-bed streams where partial bed mobility causes steep gravel transport rating curves. Qeff may approach bankfull flow in some step-pool channels where gravel moves over a static cobble/boulder bed. Our conclusions are based on magnitude-frequency analyses conducted at 41 gauged Rocky Mountain headwater streams. Because these gauged streams lacked gravel transport data, as is typical, comparable streams with measured transport rates were used to develop scaling relations for rating curve exponents with stream and watershed characteristics. Those scaling relations were then used to estimate the steepness of gravel rating curves at the 41 gauged but unsampled sites. The measured flow frequency distributions were characterized by two fitted power functions. The steepness of the flow frequency distributions and the estimated steepness of gravel transport relations were combined in magnitude-frequency analyses to compute Qeff.