Effective discharge in small formerly glaciated mountain streams of British Columbia: Limitations and implications

Abstract

Episodic sediment supply, past glaciation, and slow responses to disturbance make small mountain streams transitional alluvial regimes in which nonequilibrium conditions are common. Bed load effective discharge in these streams is on average a low-magnitude, high-frequency event, but is highly variable. Using a two-phase sediment transport model and long-term discharge records, we distinguish between three types of streams; streams in which gravel (sediment > 8 mm diameter) moves frequently and effective discharge occurs during gravel transport (Frequently Mobile Gravel (FMG)), streams in which gravel moves infrequently but effective discharge nonetheless occurs during gravel transport (Infrequently Mobile Gravel (IMG)), and streams in which sand (sediment < 8 mm diameter) moves over largely immobile gravel and effective discharge occurs frequently during sand-phase transport (Sand over Immobile Gravel (SG)). Using only effective discharge frequency or magnitude to characterize a stream, without information on mobile sediment type, is insufficient to distinguish between FMG and SG streams. Only the IMG streams have large, rare effective discharges that approximate the bankfull discharge; in FMG and SG streams the effective discharge is much more frequent and smaller than the bankfull. Only in the IMG streams does the effective discharge approximate a channel-forming discharge. In FMG and SG streams, the effective discharge bears little relation to the size or dimensions of the channel and is at best a channel-maintaining flow; at worst it is geomorphically meaningless. Effective discharge should not therefore be used in isolation as a proxy for channel-forming discharge for mountain stream channel design or management.