Table 1. Summary of hydrologic parameters used in IHA and their ecological influences.

Adapted from IHA Manual V7 (28).

IHA statistics group	Regime characteristics	Ecosystem influences
Group 1: Magnitude of monthly water conditions (12 indices)	Mean or median value for each calendar month	Habitat availability for aquatic organisms Soil moisture availability for plants Availability of water for terrestrial animals Availability of food/cover for furbearing mammals Reliability of water supplies for terrestrial animals Access by predators to nesting sites Water temperature, oxygen levels, and photosynthesis in water column
Group 2: Magnitude and duration of annual extreme water conditions (12 indices)	Annual minima, 1-day means Annual minima, 3-day means Annual minima, 7-day means Annual minima, 30-day means Annual minima, 90-day means Annual maxima, 1-day means Annual maxima, 3-day means Annual maxima, 7-day means Annual maxima, 30-day means Annual maxima, 90-day means Number of zero flow days Base flow index	Balance of competitive, ruderal, and stress-tolerant organisms Creation of sites for plant colonization Structuring of aquatic ecosystems by abiotic versus biotic factors Structuring of river channel morphology and physical habitat conditions Soil moisture stress in plants Dehydration in animals Anaerobic stress in plants Volume of nutrient exchanges between rivers and floodplains Duration of stressful conditions such as low oxygen and concentrated chemicals in aquatic environments Distribution of plant communities in lakes, ponds, and floodplains Duration of high flows for waste disposal and aeration of spawning beds in channel sediments
Group 3: Timing of annual extreme water conditions (2 indices)	Julian date of each annual, 1-day maximum Julian date of each annual, 1-day minimum	Compatibility with life cycles of organisms Predictability/avoidability of stress for organisms Access to special habitats during reproduction or to avoid predation Spawning cues for migratory fish Evolution of life history strategies and behavioral mechanisms
Group 4: Frequency and duration of high and low pulses (4 indices)	Number of low pulses within each water year Mean or median duration of low pulses (days) Number of high pulses within each water year Mean or median duration of high pulses (days)	Frequency and magnitude of soil moisture stress for plants Frequency and duration of anaerobic stress for plants Availability of floodplain habitats for aquatic organisms Nutrient and organic matter exchanges between river and floodplain Soil mineral availability Access for waterbirds to feeding, resting, and reproduction sites Bed load transport, channel sediment textures, and duration of substrate disturbance (high pulses)
Group 5: Rate and frequency of water condition changes (3 indices)	Rise rates: Mean or median of all positive differences between consecutive daily values Fall rates: Mean or median of all negative differences between consecutive daily values Number of hydrologic reversals	Drought stress on plants (falling levels) Entrapment of organisms on islands and floodplains (rising levels) Desiccation stress on low-mobility stream edge organisms