More than 100 waterbodies in southern California have been designated as impaired for their beneficial uses under Section 303(d) of the Clean Water Act for a range of constituents. Despite the number of impaired waterbodies, currently there is no basis for differentiating water quality problems from natural variability. Without knowing the range of natural background levels, it is difficult to discern whether high levels of naturally occurring constituents indicate a pollution problem. Furthermore, lack of information on background concentrations, load, and flux complicates determination of appropriate management targets when remediating impaired waterbodies. To fully evaluate the effect of anthropogenic activities, it is important to describe water quality in streams draining natural environments and to understand the factors that control these natural loadings. The overall goal of this study is to evaluate the water quality contributions and properties of stream reaches in natural catchments throughout southern California.