Geomorphic response of low-gradient, meandering and braided alluvial river channels to increased sediment supply

Alteration to the sediment supply of fluvial channels is well understood to trigger morphological adjustment, but the particularities and peculiarities that arise in any specific situation – the magnitude and direction of change that occurs in response to a given sediment supply modification – have long complicated broad conceptualizations of channel evolution. Because many projected changes due both to climate and human actions stand to increase sediment supplied to world rivers, it is imperative that our understanding of river response to influxes of sediment be comprehensive, underpinned by sound theory and augmented by empirical observations. To that end, here we review the literature concerning the geomorphic response of low-gradient, braided and meandering alluvial river channels to increases in sediment supply. We define such rivers as those with a gradient <0.002 that flow within a self-formed floodplain and are semi-confined or unconfined in terms of capacity for lateral adjustment. The intent of this review is both to provide managers, practitioners, and scientists with a readily referenceable summary of changes observed in a multitude of settings and to create an empirically supported conceptual framework that can be used in concert with existing models to contextualize and anticipate geomorphic response to a sediment influx. Upon analysis, several ideas emerge from the literature. First, channel adjustment following a sediment supply increase occurs across a series of nested response scales, which are separated by thresholds defined by aspects of the sediment influx. Second, the crossing of these thresholds is primarily a function of the influx grain size distribution and volume relative to the antecedent bed grain size and sediment load. Third, we term these thresholds, which represent the sediment input parameters necessary to drive change to occur at successively larger scales, influx effectiveness thresholds. Last, future work must focus on further quantification of these thresholds by explicit measurement of various metrics, including influx volumes and characteristics and antecedent sediment loads and parameters, and examination of additional controlling factors such as system trajectory and biotic influences. The synthesis and framework presented here should ideally assist in enhanced understanding and improved prediction of geomorphic adjustment following a sediment supply increase as well as steer future studies towards thorough investigations of the various dynamics necessary to further increase our knowledge of channel response(s) to changing sediment inputs.