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

Pools and riffles are alternating topographic lows and highs that are ubiquitous in gravel-bed rivers. They create a mosaic of aquatic habitats and have therefore been central to the design of many river restoration initiatives. Despite extensive research, there remains much uncertainty about the processes governing their long-term evolution, while field identification remains constrained by subjective, flow-dependent criteria.

We present an objective and reproducible framework for delineating pools directly from bed topography. The method was applied to a 45-year record of annual topographic surveys at Carnation Creek, British Columbia. Our results indicate that pools persist in channel narrowing sections and become increasingly transient where the channel widens. The most persistent pool exhibited four distinct morphological phases, marked by non-uniform adjustments in depth, area, width, and volume. Principal component analysis further reveals that these phases are embedded within broader, reach-scale elevation patterns, demonstrating how pools are dynamic yet resilient features capable of organizing reach-scale channel morphology. Their adjustment arises from recurrent cycles of aggradation and degradation driven by feedback among flow variability, sediment storage, and antecedent bed conditions.

Together, this study presents the first empirical record of the surprisingly decadal-scale persistence and adjustment of pools at an annual resolution that has not been previously reported by any field-based investigation. From a restoration perspective, these findings underscore the need to restore underlying channel processes that allow pools to self-organize, rather than imposing static, form-based designs.