Supply and Flux of Sediment Along Hydrological Pathways: Anthropogenic Influences at the Global Scale

It has become increasingly difficult to assess the impact of changes in the sediment flux to the coastal zone because of the conflicting impacts of humans. Globally, soil erosion is accelerating (e.g., deforestation, some agriculture practices), while at the same time sediment flux to the coastal zone is globally decelerating (e.g., water diversion schemes, dams). Reduced loads delivered to the coastal zone result in an accelerated coastal erosion and a decrease in habitat. The reduction of the seasonal flood waves also means that sediment is dispersed over smaller areas of the continental margin. One of the great challenges facing hydrologists, with regard to our understanding of anthropogenic versus climate-change impacts on the hydrological cycle, is the quality and duration coverage of hydrological data, particularly on pristine rivers. For many rivers, the length of hydrological coverage is measured in years rather than decades, with no rivers with reliable observations across centuries. In other words, if a river's discharge is monitored during periods where the impacts of humans or nature produce a long-term trend, then the return-interval concept for floods and droughts, for instance, becomes meaningless. In addition, river systems are conditioned by events that are largely prehistorical. Forensic studies of paleoriver systems provide us with some ability to assess and discern the impacts of humans versus climate.