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Kidder et al. to which stressful effects such as inundation or desiccation can accumulate. (5) The rate of change in water conditions may be tied to the stranding of certain organisms along the waters edge or in ponded depressions, or the abil- ity of plant roots to maintain contact with phreatic water supplies. The 32 IRA parameters provide a detailed representa- tion of the hydrologic regime for the purpose of assess- ing hydrologic alteration. Most important, they entail hy- drologic statistics commonly employed in limnology studies because of their great ecological relevance (Gus- tard 1984; Kozlowski 1984; Hughes & James 1989; Poff & Ward 1989). Also, because certain streamflow levels shape physical habitat conditions within river channels, we also identified 32 hydrologic characteristics that might aid in detection of physical habitat alteration in lotic systems. For example, changes in the central ten- dency of annual maxima might suggest changes in river morphology (Leopold 1994). Sixteen of the hydrologic characteristics focus on the magnitude, duration, timing, and frequency of extreme events because of the pervasive influence of extreme forces in ecosystems (Gaines & Denny 1993) and geo- morphology (Leopold 1994); the other 16 parameters measure the central tendency of either the magnitude or rate of change of water conditions. We describe the ra- tionale underlying the five major grouping of hydrologic characteristics and the specific parameters included within each. 1. MAGNITUDE This group includes 12 parameters, each of which mea- sures the central tendency (mean) of the daily water conditions fora given month. The monthly mean of the daily water conditions describes "normal" daily condi- tions for the month, and thus provides a general mea- sure of habitat availability or suitability. The similarity of monthly means within a year reflects conditions of rela- tive hydrologic constancy, whereas inter-annual varia- tion (e.g., coefficient of variation) in the mean water condition for a given month provides an expression of environmental contingency (Colwell 1974; Poff & Ward 1989). The terms constant}, and contingence as used here refer to the degree to which monthly means vary from month to month (constancy) and the extent to which flows vary within any given month (contin- gency). 2. MAGNITUDE AND DURATION OF ANNUAL EXTREME CONDITIONS The 10 parameters in this group measure the magnitude of extreme (minimum and maximum) annual water con- ditions of various duration, ranging from daily to sea- sonal. The durations we used follow natural or human- Assessing 1 ),drologicAlleralion 1167 imposed cycles and include the 1-day, 3-day, 7-day (weekly), 30-day (monthly), and 90-day (seasonal) ex- tremes. For any given year, the 1-day maximum (or mini- mum) is represented by the highest (or lowest) single daily value occurring during the year; the multi-day max- imum (or minimum) is represented by the highest (or lowest) multi-day average value occurring during the year. The mean magnitude of high and low water ex- tremes of various duration provide measures of environ- mental stress and disturbance during the year; con- versely, such extremes may be necessary precursors or triggers for the reproduction of certain species. The in- ter-annual variation (e.g., coefficient of variation) in the magnitude of these extremes provides another expres- sion of contingency. 3. TIMING OF ANNUAL EXTREME CONDITIONS Group 3 includes two parameters, one measuring the Julian date of the 1-day annual minimum water condi- tion and the other measuring -the Julian date of the 1-day maximum water condition. The timing of the highest and lowest water conditions within annual cycles pro- vides another measure of environmental disturbance or stress by describing the seasonal nature of these stresses. Key life-cycle phases (e.g., reproduction) may be inti- mately linked to the timing of annual extremes; thus hu- man-induced changes in timing may cause reproductive failure, stress, or mortality. The inter-annual variation in timing of extreme events reflects environmental contin- gency. 4. FREQUENCY AND DURATION OF HIGH AND LOB' PULSES The four parameters in group 4 include two that mea- sure the number of annual occurrences during which the magnitude of the water condition exceeds in upper threshold or remains below a lower threshold, respec- tively, and two that measure the mean duration of such high and low pulses (Fig. 2). These measures of fre- quency and duration of high and low water conditions together portray the pulsing behavior of environmental variation within a year and provide measures of the shape of these environmental pulses. Hydrologic pulses are defined here as those periods within a year in which the daily mean water condition either rises above the 75th percentile (high pulse) or drops below the 25th percentile (low pulse) of all daily values for the pre- impact time period. 5. RATE AND FREQUENCY OF CHANGE IN CONDITIONS The four parameters in group 5 measure the number and mean rate of both positive and negative changes in water conditions from one day to the next (Fig. 3). The rate and frequency of change in water conditions can be described in terms of the abruptness and number of in- (.I)nwr,'a 11ill Bi hnn \l,lumc I0 _ \??. ,. \u?ua 1990