In order to understand and control the impact of non-point source pollution in Central California and many semi-arid regions of the world, a multiscale observational network focused on several water quality issues is proposed. Specifically, we are targeting the main stem of the San Joaquin River as a prototypical river system. By developing multiscale modeling approaches, we will endeavor first to understand the hydrodynamics of salt mixing on the San Joaquin at each of the three river confluences (with the Merced, Tuolumne, and Stanislaus Rivers) and create salt-flow rating curves characterizing 5 different flow conditions, and second, to quantify and sample groundwater-surface water interactions along critical reaches of the river where the US Geological Survey is investigating with respect to groundwater accretion.