Geomorphic Controls on Sediment Mobility and Channel Stability of a Riffle-Pool Gravel Bed Channel

Abstract

Riffle-pool morphologies are common in gravel bed streams yet the conditions and feedbacks that maintain them remain poorly understood. In this paper, we examine temporal and spatial patterns of channel adjustment, sediment mobility and geomorphic stability of a riffle-pool reach to changes in flow and sediment supply regimes. To achieve our goals, we use a longitudinal monitoring campaign of 12 years of field measurements and flume experiments. The channel adjusts to flow and sediment supply regimes both vertically (erosion/deposition) and laterally, resulting in sediment movement through the reach while the riffle-pool morphology persists. This persistence suggests that the riffle-pool morphology is dynamically stable and is maintained through adjustment of internal processes. To better understand the evolution of stream channels with riffle-pool morphologies responding to changes in sediment supply and flow regime, we conducted a flume experiment with four runs. Our field and experimental results show that in a natural riffle-pool morphology changes in flow rates and sediment input can drive local changes in bed topography and cause dynamic trends in sediment transport volume and texture. However, riffle-pool sequences possess clear evidence of geomorphic stability as the morphological sequence and character persists under a range of conditions. These sequences are also critical for determining particle mobility and transport in a wide range of flows and sediment supply regimes.KeywordsRiffle-poolGravel-bed streamSediment transportGeomorphic stabilitySediment supplyFlow regime