Monitoring suspended sediment and associated trace element and nutrient fluxes in large river basins in the USA

In 1996, the US Geological Survey converted its occurrence and distribution-based National Stream Quality Accounting Network (NASQAN) to a national, flux-based water-quality monitoring programme. The main objective of the revised programme is to characterize large USA river basins by measuring the fluxes of selected constituents at critical nodes in various basins. Each NASQAN site was instrumented to determine daily discharge, but water and suspended sediment samples are collected no more than 12-15 times per year. Due to the limited sampling programme, annual suspended sediment fluxes were determined from site-specific sediment rating (transport) curves. As no significant relationship could be found between either discharge or suspended sediment concentration (SSC) and suspended sediment chemistry, trace element and nutrient fluxes are estimated using site-specific mean or median chemical levels determined from a number of samples collected over a period of years, and under a variety of flow conditions.