Hydrologic classifications unveil the structure of relationships among groups of streams with differing stream flow and provide a foundation for drawing inferences about the principles that govern those relationships. Hydrologic classes provide a template to describe ecological patterns, generalize hydrologic responses to disturbance, and stratify research and management needs applicable to ecohydrology. We developed two updated hydrologic classifications for the continental US using two streamflow datasets of varying reference standards. Using only reference-quality gages, we classified 1715 stream gages into 12 classes across the US. By including more streamflow gages (n=2618) in a separate classification, we increased the dimensionality (i.e. classes) and hydrologic distinctiveness within regions at the expense of decreasing the natural flow standards (i.e. reference quality). Greater numbers of classes and higher regional affiliation within our hydrologic classifications compared to that of the previous US hydrologic classification (Poff, 1996) suggested that the level of hydrologic variation and resolution was not completely represented in smaller sample sizes. Part of the utility of classification systems rests in their ability classify new objects and stratify analyses. We constructed separate random forests to predict hydrologic class membership based on hydrologic indices or landscape variables. In addition, we provide an approach to assessing potential outliers due to hydrologic alteration based on class assignment. Departures from class membership due to disturbance take into account multiple hydrologic indices simultaneously; thus, classes can be used to determine if disturbed streams are functioning within the realm of natural hydrology.
