Channel-Forming Discharge Selection in River Restoration Design

The concept of channel-forming $\left(Q_{\mathrm{cf}}\right)$ or dominant discharge is now a cornerstone of river channel restoration design. Three measures of channel-forming discharge are most commonly applied: effective discharge $\left(Q_{\mathrm{eff}}\right)$ , bankfull discharge $\left(Q_{\mathrm{bf}}\right)$ , and a discharge of a certain recurrence interval $\left(Q_{\mathrm{ri}}\right)$ , which theoretically are similar in geomorphically stable channels. The latter two measures have become particularly widely applied in some channel restoration design procedures, often to the exclusion of $Q_{\mathrm{eff}}$ analyses, despite the additional utility of $Q_{\mathrm{eff}}$ analysis for most channel design problems. We quantify the three measures of $Q_{\mathrm{cf}}$ for four case studies and then follow this with a synthesis of previously published studies to illustrate sources of variability. This synthesis suggests that agreement among the three measures of $Q_{\mathrm{cf}}$ is best for snowmelt-hydrology, nonincised channels with coarse substrate. Departures from these conditions result in greater discrepancy between the measures. Channel incision produces $Q_{\mathrm{bf}}$ far greater than $Q_{\mathrm{eff}}$ , and flashy hydrology is associated with generally larger, briefer, and more frequent $Q_{\mathrm{eff}}$ . Regional mean or median values for the relative magnitudes of the three measures can be tightly constrained, but site to site variation is quite large. The construction of a cumulative sediment discharge curve and associated determination of $Q_{\mathrm{eff}}$ allows quantification of the sediment budget of a channel for a given hydrologic regime, which provides process-based insight of drivers of current and future trajectories of channel stability, and is thus the recommended measure of channel-forming discharge. Reliance on only return-interval or bankfull discharge for channel design is not recommended for channel design activities.