Effects of sediment pulses on bed relief in bar-pool channels

Sediment pulses, such as those derived from dam removal or landsliding, can modify habitat by filling pools and thereby homogenizing the channel bed and therefore represent a primary concern for managers, and we need the tools and theory to simply anticipate the range of morphological adjustments of a bar-pool channel to changes in sediment thickness. We quantify channel morphology as cross-channel bed relief and develop a new analytical framework representing bed relief as a function of sediment thickness, where both quantities at a cross-section are normalized by the 90th percentile of observed bed relief values within a reach prior to disturbance. We employ the new framework and data downstream of two dam removals in Oregon, Brownsville Dam on the Calapooia River and Marmot Dam on the Sandy River, to test the hypothesis that, with respect to potentially sediment-deficient pre-disturbance channel cross-sections, bed relief initially increases with increasing sediment thickness due to the concentration of deposition within bar forms, reaches a maximum corresponding to the maximum height of bars, and subsequently decreases as more sediment is added due to the concentration of deposition within bar-adjacent pools. Results show that bed relief values do initially increase with increasing sediment thickness and may subsequently decrease with larger thickness, at least on average, but the more marked result is the 150% increase in the range of values of bed relief with increased sediment thickness, a range that suggests greater habitat heterogeneity with additional sediment thickness for the small- to moderate-sized sediment pulses studied.