River networks are important controllers of material transfer from land to the ocean. Understanding the factors regulating this function for different gaseous, dissolved, and particulate constituents is critical to assess the local and global effects of climate and land use change. We propose the River Network Saturation hypothesis to explain how river network regulation of material fluxes changes with flow conditions due to imbalances between constituent supply and demand at network scales. In contrast to terrestrial ecosystems, saturation of river networks is highly variable in time due to the considerable variation in the supply of constituents associated with changes in flow. We used simple river network models to explore environmental factors that influence river network saturation.
Results/Conclusions
All river networks become saturated under high flow conditions, but flow thresholds under which saturation occurs depends on the inherent process rates for a given constituent, saturating kinetics, and the abundance of lentic waters within the river network. As supply increases, saturation at network scales is initially limited by previously unmet demand in downstream aquatic ecosystems. Four stages of network-scale constituent removal describe the saturation response of an entire river network. Stage 1 is characterized by 100% removal at network scale because demand is so large compared to supply that constituents are immediately processed as they enter the network. Stage 2 continues to show near complete removal at the network scale, mostly because retention by downstream reaches prevents any leakage from the overall network. Stage 3 is characterized by rapid declines in the proportion of constituent removed, resulting in increased breakthrough and export from the river network as loads continue to increase with a slowing increase of the commensurate demand. In Stage 4, the river network essentially has little or no attenuation of input fluxes, because supply overwhelms demand. Better understanding of when and where river networks saturate for different constituents will allow extrapolation of aquatic function to broader spatial scales and help identify management priorities.