constitutes the watershed. Overall, the channels and watershed landscapes of most of the nation’s rivers have been so modified that the concept of a “natural river system” sensu stricto reference state may no longer exist.

Despite this difficult context, many of today’s environmental policy decisions call for science-based information about the likely responses of river systems to changes in both natural forcing and human drivers. Empirical equations have helped improve the hydrological science knowledge of these responses. Generally speaking, regression-based approaches have been extremely useful in determining the likely responses within a defined context. The predictive abilities of these descriptive equations are, however, limited when the environment exceeds the normal bounds. The most challenging problems demand information about river systems’ responses to future conditions that are outside the range of historical observations and experience, such as extreme events like Hurricane Katrina.

River science must, therefore, be structured and conducted to provide a process-based and predictive understanding of river systems. This understanding must go beyond methods that have commonly been used in the past to guide policy and management decisions, that is, operational or pragmatic predictions based on empirical associations such as regression relationships. Rather, these associations should be used to help uncover the processes beneath the trends. Sound policy decisions require a sufficient understanding of river systems so as to offer sound, credible, testable predictions of river systems’ responses to new and previously unobserved forcing that could accompany climate change, excessive groundwater extractions, large-scale land-use conversion, hydrologic alterations from urbanization and stormwater management and, ironically, restoration actions designed to mitigate or reverse some of the above forcings.

CONCLUSION AND RECOMMENDATION

The understanding necessary to assess the complex changes of river systems to management alternatives will not come easily, especially in the midst of uncertainty. Therefore, there is a compelling national need for a new approach to studying rivers. River science—an emerging discipline distinct from traditional disciplinary sciences but still supported by their activities—can provide a vision for organizing scientific endeavors to address these unique challenges.


Recommendation: USGS river science activities should be driven by the compelling national need for an integrative multidisciplinary science structured and conducted to develop a process-based predictive understanding of the functions of the nation’s river systems and their responses to natural variability and the growing, pervasive, and cumulative effects of human activities.



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