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Figure 1. Flow regime is of central importance Flow Regime in sustaining the eco- Magnitude logical integrity of flow- Frequency ing water systems. The Duration Timing five components of the Rate of Change flow regime-magni- tude, frequency, dura- tion, timing, and rate rate of change-influence integrity both directly Water Energy Physical Biotic and indirectly, through Quality Sources Habitat interactions their effects on other primary regulators of integrity. Modification of flow thus has cas- cading effects on the ecological integrity of Ecological Integrity rivers. After Karr 1991. Wildlife Service for water-dependent species of sporting, commercial, or conservation value), making it diffi- cult, if not impossible, to manage the entire river ecosystem (Karr 1991). However, environmental dynamism is now recognized as central to sus- taining and conserving native spe- cies diversity and ecological integ- rity in rivers and other ecosystems (Holling and Meffe 1996, Hughes 1994, Pickett et al. 1992, Stanford et al. 1996), and coordinated actions are therefore necessary to protect and restore a river's natural flow variability. In this article, we synthesize exist- ing scientific knowledge to argue that the natural flow regime plays a critical role in sustaining native biodiversity and ecosystem integrity in rivers. Decades of observation of the effects of human alteration of natural flow regimes have resulted in a well- grounded scientific perspective on why altering hydrologic variability in rivers is ecologically harmful (e.g., Arthington et al. 1991, Castleberry et al. 1996, Hill et al. 1991, Johnson et al. 1976, Richter et al. 1997, Sparks 1995, Stanford et al. 1996, Toth 1995, Tyus 1990). Current pressing demands on water use and the continuing alter- ation of watersheds require scientists to help develop management proto- cols that can accommodate economic uses while protecting ecosystem func- tions. For humans to continue to rely on river ecosystems for sustainable food production, power production, waste assimilation, and flood con- trol, a new, holistic, ecological per- spective on water management is needed to guide society's interac- tions with rivers. The natural flow regime The natural flow of a river varies on time scales of hours, days, seasons, years, and longer. Many years of observation from a streamflow gauge are generally needed to describe the characteristic pattern of a river's flow quantity, timing, and variability- that is, its natural flow regime. Com- ponents of a natural flow regime can be characterized using various time series (e.g., Fourier and wavelet) and probability analyses of, for example, extremely high or low flows, or of the entire range of flows expressed as average daily discharge (Dunne and Leopold 1978). In watersheds lacking long-term streamflow data, analyses can be extended statisti- cally from gauged streams in the same geographic area. The frequency of large-magnitude floods can be es- timated by paleohydrologic studies of debris left by floods and by studies of historical damage to living trees (Hupp and Osterkamp 1985, Knox 1972). These historical techniques can be used to extend existing hydrologic records or to provide estimates of flood flows for ungauged sites. River flow regimes show regional patterns that are determined largely by river size and by geographic varia- tion in climate, geology, topogra- phy, and vegetative cover. For ex- ample, some streams in regions with little seasonality in precipitation ex- hibit relatively stable hydrographs due to high groundwater inputs (Fig- ure 2a), whereas other streams can fluctuate greatly at virtually any time of year (Figure 2b). In regions with seasonal precipitation, some streams are dominated by snowmelt, result- ing in pronounced, predictable run- off patterns (Figure 2c), and others lack snow accumulation and exhibit more variable runoff patterns during the rainy season, with peaks occur- ring after each substantial storm event (Figure 2d). Five critical components of the flow regime regulate ecological pro- cesses in river ecosystems: the mag- nitude, frequency, duration, timing, and rate of change of hydrologic conditions (Poff and Ward 1989, Richter et al. 1996, Walker et al. 1995). These components can be used to characterize the entire range of flows and specific hydrologic phe- nomena, such as floods or low flows, that are critical to'the integrity of river ecosystems. Furthermore, by defining flow regimes in these terms, the ecological consequences of par- ticular human activities that modify one or more components of the flow regime can be considered explicitly. • The magnitude of discharge' at any given time interval is simply the amount of water moving past a fixed location per unit time. Magnitude can refer either to absolute or to relative discharge (e.g., the amount of water that inundates a floodplain). Maximum and minimum magnitudes of flow vary with climate and water- shed size both within and among river systems. • The frequency of occurrence refers to how often a flow above a given magnitude recurs over some speci- fied time interval. Frequency of oc- currence is inversely related to flow magnitude. For example, a 100-year flood is equaled or exceeded on aver- age once every 100 years (i.e., a chance of 0.01 of occurring in any given year). The average (median) 'Discharge (also known as streamflow, flow, or flow rate) is always expressed in dimen- sions of volume per time. However, a great variety of units are used to describe flow, depending on custom and purpose of charac- terization: Flows can be expressed in near- instantaneous terms (e.g., ft'/s and m'/s) or over long time intervals (e.g., acre-ft/yr). 770 BioScience Vol. 47 No. 11