managed as de minimis local perturbations. Yet from the ubiquitous presence of pharmaceuticals in natural waters to the wholesale diversion and consumptive use of water resource systems, the cumulative impacts of local management decisions have surpassed anticipated consequences. Furthermore, they confound the traditional management frameworks that guide policy and management decisions. Therefore, the national need for a new integrated multidisciplinary river science is more compelling now than ever before.

Within this chapter, we explore these challenges that are driving our current policies and management practices and thus our need for river science: ecological restoration (including dam removal), relicensing of hydropower facilities, invasive species, water allocation, climatic variability, urbanization and other land-use changes, and water quality. We also mention briefly the economic value of river ecosystem services, a matter that is particularly relevant when the case for river science is rationalized based on the values rivers provide. We conclude with an outline of the characteristics that river science needs to have to confront the individual unmet challenges and overall cumulative effects that human activities have on river ecosystems.

ECOLOGICAL RESTORATION AND DAM REMOVAL

Human and natural actions have caused the loss or degradation of riverine habitat. Throughout the country, thousands of ecological restoration efforts are being undertaken to improve water quality, manage riparian zones, improve habitat, and stabilize streambanks (Bernhardt et al., 2005). Billions of dollars are being spent on small projects including an Ecosystem Initiative on the Platte River (Box 2-1) and billions more on major restoration projects in the Everglades, coastal Louisiana, the California Bay and Delta, and the upper Mississippi River. Yet the science of river restoration is still in its infancy.

Dam removal is a high-profile form of river restoration. Because dams with significant storage capacity dramatically alter riverine flows—creating lakes where rivers once flowed and fundamentally altering the downstream flow regime—they have had enormous impacts on ecological patterns and processes in rivers. Currently in the United States, the National Inventory of Dams lists 76,000 dams that exceed 2 meters in height and estimates 2 million more of less than 2 meters in height (Graf, 1999; http://crunch.tec.army.mil/nid/webpages/nid.cfm). For many of these dams, their original uses have long disappeared and they stand only to hold back river water and are thus a repository for collecting river sediments, nutrients, and contaminants. Almost 500 dams were removed during the 20th century, most of which were less than 5 meters in height (Poff and Hart, 2002). Removal of these small dams has been accelerating in recent years as their economic viability declines, dam owners realize their increasing legal and financial liabilities, and the government recognizes the environmental benefits of their removal.



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