Background/Question/Methods

Streams and rivers are highly variable systems that are strongly regulated by hydrologic disturbance.  The magnitude, frequency, duration and timing of disturbance flows strongly constrain ecological processes and species-to-ecosystem characteristics of riverine systems.  Variation in these components of the so-called natural flow regime occurs geographically along prevailing climatic (precipitation) gradients.  The natural flow regime provides a template against which ecological characteristics are expressed.  Through building of dams or construction of levees, humans have modified flow regimes and disrupted the longitudinal and lateral connectivity of rivers.  Such hydrologic alteration has modified many ecosystem goods and services in river channels and on adjacent floodplains.  Climate change is likely to bring substantial changes in the frequency and magnitude and, in some cases, the form of precipitation.  These changes need to be considered in the context of deviation from a prevailing disturbance regime in order to understand the ecosystem adjustments that may result. 

Results/Conclusions

The existence of multi-decadal stream gauging records allows the natural flow regime of rivers across the U.S. to be defined in recent historical time.  Examples of natural flow regimes in unregulated rivers are given across the U.S.  The extent to which these flow regimes have been modified by human activities (dams) is quantified.  The region-specific changes in disturbance regimes associated with river regulation during the 20^th Century are shown to be much greater than decadal-scale changes caused by climate variation during that period, suggesting an important role of human management for desired ecosystem states in riverine systems.  Key linkages between components of hydrologic variation and ecological processes that underlie the provision of natural goods and services and native biodiversity are discussed and placed in a regional context.  The vulnerability of river systems to region-specific changes in climate-driven environmental variation is discussed and opportunities for sustainable management are explored.