When pools cool streams: A dimensionless framework for reach-scale thermal buffering

Pools are fundamental morphologic features in streams and rivers that provide key habitats for native aquatic species to forage, avoid predation, and shelter from adverse hydraulic and thermal conditions. Here we develop a physically-based dimensionless framework linking pool volume and flow hydraulics to reach-scale thermal buffering of streamwater temperature owing to pool stratification by modeling the process with a transient-storage model. The framework identifies two dimensionless quantities: A^⁎, the ratio of downstream to upstream daily water temperature amplitudes that quantifies the magnitude of thermal buffering, and P^⁎, a new river-pool coupling predictor that combines the river-to-pool volume ratio and a hydraulic mixing number to quantify the strength of thermal coupling between pools and the main river flow. We test the framework on the Bird Track Springs reach of the Grande Ronde River in Oregon (USA), comparing pre-restoration conditions with a single small pool to post-restoration conditions with sixteen constructed pools under similar valley conditions. Field data reveal that when discharge decreases below a threshold value of P^⁎≈0.014, pools begin to buffer diel temperature fluctuations by up to 80 %. The framework explains how the measurable parameters of morphologic storage (residual pool volume), stream discharge, and hydraulic mixing jointly control reach-scale temperature variability. The framework provides a simple, transferable tool for 1) evaluating natural system thermal buffering capacity and 2) predicting and designing restoration actions that sustain thermal refuges and mitigate heat stress in riverine environments.