River channel and watershed self-organization limit the flux of fine particles

Abstract:

Fine particles in rivers represent significant water quality concerns due to their potential to both degrade and enhance aquatic habitats as well as their role as carriers of nutrients and contaminants. Observed and modeled increases in intense precipitation due to climate change will exacerbate erosion increasing the supply of fine particles within watersheds. Unfortunately, predicting fine particle dynamics within rivers has remained challenging due to a complex covarying relationship between particulate and water availability within the watershed. This inherent complexity results in watershed-specific understandings of suspended particle dynamics, typically parameterized as empirical functions of discharge, land use, lithology, and climate. However, observations of significant fine particle storage within river corridors indicates that the flux of suspended sediment may depend on reach-scale hydraulics, which self-organize based on regional hydroclimate and channel material properties to limit transport capacity. These observations provide a physical framework from which fine particle dynamics may be interpreted. We synthesized over 40 years of US Geological Survey suspended sediment concentration (SSC) data across 1,148 rivers revealing that SSC have a common probability distribution despite a large variety of environments. Within a subset of these data (104 rivers) we’ve assembled detailed analyses of SSC, hydraulic geometry, river flow, and grainsize data to test physically based frameworks of suspended particle transport. This data synthesis reveals a parsimonious physically based nonlinear trend between reach-scale hydraulics and the local suspended sediment flux indicating a larger role for in and near channel storage of fine particles. This general relationship demonstrates that suspended sediment is more predictable than previously believed and highlights the role of storage in homogenizing the signals of watershed erosion processes. However, this analysis also indicates that many of these rivers are transporting the finest particles well under capacity, highlighting an emerging management priority as increased watershed erosion due to intense precipitation has the potential to drastically reduce water quality.