Bankfull and effective discharge in small mountain streams of British Columbia

• Main Author: Brayshaw, Drew Devoe
• Language: English
• Published: University of British Columbia 2012
• Online Access: http://hdl.handle.net/2429/40245

In this thesis, the concept of channel-forming discharge developed for large lowland rivers is critically re-evaluated for small mountain streams. In large lowland rivers under equilibrium conditions, the effective discharge (the discharge interval that transports the greatest proportion of sediment) approximates the bankfull discharge. The effective and bankfull discharges therefore provide measurable analogues for the theoretical channel-forming discharge, responsible for the form and dimensions of the stream channel. In small mountain streams this concordance of bankfull and effective discharges has been suspected to break down, in part because the channel form and dimensions are determined by the non-fluvial sediment supply as well as by fluvial processes. This interaction of non-fluvial sediment supply with fluvial processes makes the application of models and concepts developed for lowland rivers to mountain streams difficult. The history of past glaciation in mountain environments adds complexity in the form of increased sediment supply and changes in streamflow regime over time. Bankfull and effective discharges are measured and estimated for small mountain streams in three distinct hydroclimatic zones in southern British Columbia. These discharges are determined using surveyed stream cross-sections, photogrammetric analysis of in-channel sediment, long-term Water Survey of Canada gauging records, and a two-fraction sediment transport model. Results indicate multiple orders of magnitude of variability in the frequency and magnitude of bankfull and effective discharge for the studied streams, and little correlation between the bankfull and effective discharges. Bankfull frequency can be related to hydraulic and hydroclimatic variables while effective discharge frequency cannot. Most of the studied streams are incised, with bankfull discharges one or more orders of magnitude greater than their effective discharge, a condition attributable to the legacy of Pleistocene glaciation. In these streams the effective discharge is not a channel-forming discharge; at best, it is a channel-maintaining flow, and at worst it is geomorphically meaningless. In a few streams, the bankfull and effective discharges are both large, rare events; these are the only streams in the study in which the bankfull and effective discharges approximate a truly channel-forming discharge.