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Richter et al. sumed perturbations (such as dam operations, flow di- version, or intensive conversion of land uses in a water- shed) by comparing the hydrologic regimes from pre- impact and post-impact time frames. We then illustrate the application of this method with a case study from the dam-altered Roanoke River in North Carolina (U.S.A). Our intent is to make available to ecosystem managers and researchers an easily utilized analytical tool for com- prehensively summarizing complex hydrologic variation with biologically relevant attributes. It is not our intent to describe or predict biological responses to hydrologic alteration. Instead, we hope that this tool will facilitate investigations into the effects of hydrologic modifica- tions on the biotic composition, structure, and function of aquatic, riparian, and wetland ecosystems. Methods Indicators of Hydrologic Alteration Our general approach for hydrologic assessment is first to define a series of biologically relevant hydrologic at- tributes that characterize intra-annual variation in water conditions and then to use an analysis of the inter-annual variation in these attributes as the foundation for com- paring hydrologic regimes before versus after a system has been altered by various human activities. Because the proposed method results in the computation of a AssessingHPdrologicAlteration 1165 representative, multi-parameter suite of hydrologic char- acteristics-or indicators-for assessing hydrologic al- teration, we refer to it as the Indicators of Hydrologic Al- teration (IHA) method. The IRA method has four steps: (1) Define the data series (e.g., stream gauge or well records) for pre- and post-impact periods in the eco- system of interest. (2) Calculate values of hydrologic attributes. We calcu- late values for each of 32 ecologically relevant hy- drologic parameters (Table 1) for each year in each data series, i.e., one set of values for the pre-impact data series and one for the post-impact data series. (3) Compute inter-annual statistics. We compute mea- sures of central tendency and dispersion for the 32 parameters in each data series, based on the values calculated in step 2. This produces 64 inter-annual statistics for each data series (32 measures of cen- tral tendency and 32 measures of dispersion). (4) Calculate values of the IHX.7"We compare the 64 in- ter-annual statistics between the pre- and post-im- pact data series, and we present each result as a percentage deviation of one time period (the post- impact condition) relative to the other (the pre- impact condition). The method can be used to compare the state of one system to itself over time (e.g., pre- versus post-impact as just described), or -it can be used to compare the state of one system to another (e.g., an altered system to a reference Table 1. Summary of hydrologic parameters used in the Indicators of Hydrologic Alteration and their characteristics. Regime IHA statistics group characteristics Hydrologic parameters Group 1: Magnitude of monthy water Magnitude mean value for each calendar month conditions Timing Group 2: Magnitude and duration of annual Magnitude Annual minima 1-day means extreme water conditoos Duration Annual maxima 1-day means Annual minima 3-day means Annual maxima 3-day means Annual minima 7-day means Annual maxima 7-day means Annual minima 30-day means Annual maxima 30-day means Annual minima 90-day means Annual maxima )o-day means Group 3: Timing of annual extreme water Timing Julian date of each annual 1 day maximum conditions Julian date of each annual 1 day minimum Group 4: Frequency and duration of high Magnitude No. of high pulses each year and low pulses Frequency No. of low pulses each year Duration Mean duration of high pulses within each year Mean duration of low pulses within each year Group 5: Rate and frequency of water Frequency condition changes Rate of change Means of all positive differences between consecutive daily means Means of all negative differences between consecutive daily values No. of rises No. of falls