Hydrologic regimes play a major role in determining the biotic composition, structure ad function of aquatic, wetland and riparian ecosystems. But human land and water uses are substantially altering hydrologic regimes around the world. Improved quantitative evaluations of human-indiced hydrologic changes are needed to advance research on the biotic implications of hydrologic alteration and to support ecosystems management and restoration plans. We propose a method for assessing the degree of hydrologic alteration attributable to human influence within an ecosystem. This method, referred to as "Indicators of Hydrologic Alteration," is based upon an analysis of hydrologic data available either from existing measurements points within an ecosystem (such as stream gauges or wells) or model-generated data. This method is intended for use with other ecosystem metrics in inventories of ecosystem integrity, in planning ecosystem management activities, and in setting and measuring progress toward conservation or restoration goals.