Pool evolution in gravel-bed reaches : insights from a 45-year record at Carnation Creek, British Columbia

Pools and riffles form an alternating sequence of topographic lows and highs that are ubiquitous in gravel-bed reaches. They provide a mosaic of aquatic habitats and have therefore been central to the design of many river restoration initiatives. Despite extensive research, there remains significant uncertainty regarding the processes governing their long-term evolution. This is largely due to the scarcity of continuous field data and the reliance on subjective, flow-dependent identification methods. This study develops an objective and reproducible framework to delineate pools directly from bed topography and applies it to a 45-year record of annual surveys from Carnation Creek, British Columbia. Patterns of pool persistence and adjustment were evaluated in relation to variability in channel width, reach-scale topography, flow regime and sediment storage-transfer dynamics. A downstream pool persisted in the narrowest section of the channel in most years and migrated with shifts in channel constriction. Pool depth, volume, area, width and length exhibited non-uniform adjustments that define four distinct morphological phases. These phases coincide with recurring cycles of aggradation and degradation that redistribute sediment unevenly across the reach and are embedded within dominant modes of topographic variability identified through principal component analysis. Together, these results provide the first empirical record of pool evolution at an annual resolution over decadal timescales. Pools emerge as dynamic yet resilient features that remain anchored to local width controls while reorganizing their geometry in response to changing hydraulic and sedimentary conditions. In this context, process-based approaches to restore pool-riffle systems should be prioritized as they better accommodate their geomorphic and ecological roles than static, form-based designs.