Data queries and maps at this website present distribution according to "drainage" in an effort to correlate aquatic species locality data with natural drainage systems. We follow a system of drainage classification developed by the USGS. Referred to as Hydrologic Unit Codes (HUC), this system identifies hydrologic regions of the US at four levels. Our maps employ the 2-digit (HUC 2), 6-digit (HUC 6) and 8-digit (HUC 8) levels. Mapping in this manner indicates recorded presence of a species from at least one location within a designated drainage, but does not necessarily imply occurrence throughout that drainage.