Long-term trends of streamflow, sediment load and nutrient fluxes from the Mississippi River Basin: Impacts of climate change and human activities

Under the influences of global climate change and intense human activities, the hydrological and biogeochemical processes have been undergoing profound changes in most of the world's watersheds. However, the long-term changes in streamflow, sediment load, and nutrient fluxes, and quantitative attribution of these changes from a basin-wide perspective are incompletely understood. Here we examined the trends of streamflow, sediment load, and total nitrogen (TN), total phosphorus (TP), and dissolved inorganic carbon (DIC) fluxes in the Mississippi River Basin (MRB) and its four major tributary basins during the 1930s-2010s, and major causes for streamflow, sediment load and nutrient fluxes changes were quantified. The streamflow increased significantly (p < 0.05) in the entire basin except for the Arkansas River Basin, while the sediment load significantly decreased except in the Arkansas River Basin and Upper MRB. Generally, the TN flux reduced except for an increasing trend in the Upper MRB. While the TP flux rose except for a decrease in the Ohio River Basin, and the DIC flux increased significantly. The strength of the relationship between nutrient flux and streamflow/sediment load showed four binary patterns, and in general the nutrient fluxes were more strongly correlated with streamflow compared to sediment load. Over the past 80 years, the increasing precipitation in the MRB has made the most contribution (67 % on average) to the river discharge increase, whereas human activities contribute the most (>58 %) to the changes in the sediment load and nutrient fluxes. The sediment yield coefficient decreased linearly with the increasing impermeable area proportion, river revetment length, and dam storage capacity (p < 0.05) in the MRB. The decrease in TN flux probably resulted from sedimentation and denitrification in reservoirs and wetlands, and increases in farm fertilizer and manure use were the two main factors contributing to the increased TP flux. In addition, the agricultural practices, increased precipitation, and raised temperature jointly led to an increase in the DIC flux. This study provides quantitative results of water–sediment-nutrient variations in the entire watershed system, which facilitates development of watershed management strategies.