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O ewronnw ??nroii?nai'. rrr y?=T*'K ?? v Aoa? S P .: .. C INIIpI. NOON ?E 4 -ROOOV..L ..... low '°°"° ::. . B. ::: .:: '.:. •....;,.t:r . .. ... .....:: C E ...oon AFRD.4iO.BR F Figure 3. Stream valley cross-sections at various locations in a watershed illustrate basic principles about natural pathways of water moving downhill and human influences on hydrology. Runoff, which occurs when precipitation exceeds losses due to evaporation and plant transpiration, can be divided into four components (a): overland flow (1) occurs when precipitation exceeds the infiltration capacity of the soil; shallow subsurface stormflow (2) represents water that infiltrates the soil but is routed relatively quickly to the stream channel; saturated overland flow (3) occurs where the water table is close to the surface, such as adjacent to the stream channel, upstream of first-order tributaries, and in soils saturated by prior precipitation; and groundwater flow (4) represents relatively deep and slow pathways of water movement and provides water to the stream channel even during periods of little or no precipitation. Collectively, overland and shallow subsurface flow pathways create the peaks in the hydrograph that are a river's response to storm events, whereas deeper groundwater pathways are responsible for baseflow. Urbanized (b) and agricultural (c) land uses increase surface flow by increasing the extent of impermeable surfaces, reducing vegetation cover, and installing drainage systems. Relative to the unaltered state, channels often are scoured to greater depth by unnaturally high flood crests and water tables are lowered, causing baseflow to drop. Side-channels, wetlands, and episodically flooded lowlands comprise the diverse flood- plain habitats of unmodified river ecosystems (d). Levees or flood walls (e) constructed along the banks retain flood waters in the main channel and lead to a loss of floodplain habitat diversity and function. Dams impede the downstream movement of water and can greatly modify a river's flow regime, depending on whether they are operated for storage (e) or as "run-of-river," such as for navigation (f). rivers in accordance with both flow characteristics and the type and the availability of transportable materials. Within a river, different habitat features are created and maintained by a wide range of flows. For ex- ample, many channel and floodplain features, such as river bars and riffle- pool sequences, are formed and main- tained by dominant, or bankfull, dis- charges. These discharges are flows that can move significant quantities of bed or bank sediment and that occur frequently enough (e.g., every several years) to continually modify the channel (Wolman and Miller 1960). In many streams and rivers with a small range of flood flows, bankfull flow can build and main- tain the active floodplain through stream migration (Leopold et al. 1964). However, the concept of a dominant discharge may not be ap- plicable in all flow regimes (Wolman and Gerson 1978). Furthermore, in some flow regimes, the flows that build the channel may differ from those that build the floodplain. For example, in rivers with a wide range of flood flows, floodplains may ex- hibit major bar deposits, such as berms of boulders along the channel, or other features that are left by infrequent high-magnitude floods (e.g., Miller 1990). - Over periods of years to decades, a single river can consistently pro- vide ephemeral, seasonal, and per- sistent types of habitat that range from free-flowing, to standing, to no water. This predictable diversity of in-channel and floodplain habitat types has promoted the evolution of species that exploit the habitat mo- saic created and maintained by hy- drologic variability. For many river- ine species, completion of the life cycle requires an array of different habitat types, whose availability over time is regulated by the flow regime (e.g., Greenberg et al. 1996, Reeves et al. 1996, Sparks 1995). Indeed, adaptation to this environmental dy- namism allows aquatic and flood- plain species to persist in the face of seemingly harsh conditions, such as floods and droughts, that regularly destroy and re-create habitat elements. From an evolutionary perspective, the pattern of spatial and temporal habitat dynamics influences the rela- tive success of a species in a particu- lar environmental setting. This habi- tat template (Southwood 1977), which is dictated largely by flow regime, creates both subtle and pro- found differences in the natural his- tories of species in different segments of their ranges. It also influences species distribution and abundance, as well as ecosystem function (Poff and Allan 1995, Schlosser 1990, Sparks 1992, Stanford et al. 1996). Human alteration of flow regime changes the established pattern of natural hydrologic variation and dis- turbance, thereby altering habitat dynamics and creating new condi- tions to which the native biota may be poorly adapted. Human alteration of flow regimes Human modification of natural hy- drologic processes disrupts the dy- namic equilibrium between the move- ment of water and the movement of sediment that exists in free-flowing rivers (Dunne and Leopold 1978). This disruption alters both gross- and fine-scale geomorphic features that constitute habitat for aquatic and riparian species (Table 1). After 772 BioScience Vol. 47 No. 11