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flow is determined from a data series of discharges defined" over a specific time interval, and it has a frequency of occurrence of 0.5 (a 50% prob- ability) . -The duration is the period of time associated with a specific flow condi- tion. Duration can be defined relative to a parttcularflowevent (e.g~t a flood- plaIn may be inundated for a specific number of days by a ten-year flood), or it can be a defined as a composite expressed over a specified time period (e,g'1 the number of days "in a year when flow exceeds some value). -The timing, or predictability, of flows of defined magnitude refers to the regularity with which,they occur. This regularity can be defined for.. mally or iriformally and with refer- ence to different time scales (Poff 199 6) ~ For example, annual peak flows may occur with low seasonal predict.. ability (Figure 2b) or with high sea- sonal predictability (Figure 2c). -The rate of change, or' flashiness, refers to how quickly flow" changes from one magnitude to another. At the extremes, "flashy') streams have rapid rates of change (Figure 2b)t whereas" stable" streams have slow rates of change (Figure 2a). Hydrologic processes and the flow regime. All river flow derives. ulti- mately from precipitation. but in,any given time and place a rlverts flow is derived from some combination of surface water. soil water, and ground- water. Cllmate~ geologyt topogra- phy, soilst and vegetation help to determine both the supply- of water and the pathways by which precipi- tation reaches the channel. The wa... ter movement pathways 9-epicted in Figure 3a" illustrate why rivers in different settings have different flow regimes and why flow is variable in virtually all rivers. Collectively, over- land and shallow subsurface flow pathways create hydrogr"aph peaks, which are the river's response to storm events. By contrast, deeper groundwater pathways are respon- sible for baseflowt the form of deliv- ery during periods of little rainfalL Variability in intensitYt timingt and duration of preclpita~ion (as rain or as snow) and in the -effects of terrain. soil texturet and plant evapo- transpiration on the hydrologic cycle combine to create local and regiona~ December 1997 Yark: Replace with new Fig. 2 .~. supplied :figure 2. Flow histories based on long-term.' daily mean dl,scharge records. These histories show withIn" and among-year variation fOf (a) Augusta Creek, Mt (b) SaUlIa River, GAt (c) upper Colorado RlveI:'t CO, and (cl) South Fork of the McKenzie River, OR. Each water year begins on October 1 and ends on September 3,0. Adapted from Pqff and-Ward 1990.- ' flow patterns. For examplet high flows due to rainstorms ,may occur over periods of hours (for permeable soils) or even minutes (for imperme.. able soils), whereas snow will melt over a period of days or weeks, which slowly builds the peak snowmelt flood~ As one proceeds dowristream within a watershed, river flow reflects the sum of flow generation and rout.... ing processes operating in multiple 'small tributary watersheds. The travel time of flow down the river system, combined with nonsynchronous tribu- ta~y inputs and larger -downstream channel and floodplain storage ca- pacities, act to attenuate and to dampen flow peaks. Consequently, annual hydrographs in large streams typically show peaks created by wide.. spread storms or snowmelt events and broad seasonal influences that affect many tributaries together (Dunne and Leopold 1978). The natural flow regime organizes and defines river ecosystems a In riv.. ers, the physical structure of the en:- vironment and, thus; of the habitatt ,is ,defjned largely by physical pro- cesses) . especially the movement of "water and sediment within the chan- nel and between the channel and flood- plain. To understand the biodiversity. production, ap.d sustalnab.ility of river ~cosystems) it is necessary to appreciate the central organizing role played by a dynamically varying physical environment~ The physical habitat of a river includes sediment size and heteroge.. nefty, 'channel and floodplain mor- phology~ and other geomorphic fea- tures. These features form as the available sediment, woody debris. and other transportable materials are moved and deposited by flow. T'hust habitat conditions associated with channels and floodplains vary among 771