both individually and collectively as constraints on population growth or determinants of successful life history traits. Many aquatic organisms synchronize their life-cycle events to the occurrence of flow regime events. For example, many aquatic insects time their emergence to avoid annual bottlenecks of peak flows or droughts, and many fish time their migrations through river networks to minimize the risk of being stranded in disconnected habitats. Many fish depend on the seasonal inundation of floodplains of lowland rivers for food and reproduction, in tropical (Junk, 1984) and temperate (Gorski et al., 2010) river systems. A wide variety of terrestrial organisms may tune their distribution and behavior to these river patterns, with organisms as diverse as humans, bears, bats, lizards, spiders, and riparian plants relying on peaks in insect emergence or fish spawning to supply a substantial portion of their annual energy or nutrient demands.

Flow regimes are being affected by land use disturbance (especially dams, water withdrawal, and channelization) and climate change. As flow regimes change, the winning evolutionary strategies of the past may prove less competitive so that native species may be lost and new species may become dominant or invade aquatic ecosystems from which they were previously excluded. Although it is clear that water resource management that is more sophisticated than merely protecting minimum flows is necessary, a central challenge for understanding the modern distribution of riverine organisms (and the terrestrial organisms that rely upon them) is to determine which aspects of the hydrograph are most closely associated with the protection, maintenance, and restoration of biological communities and ecosystem processes.

Many questions remain. How do river food web networks depend on river networks? What components of a river’s hydrologic regime are essential to channel morphodynamics and river ecosystems? What is a necessary amount of sediment supply (and size of sediment) to maintain or rebuild river ecosystems? How will changes in flood frequency regimes or extended periods of drought flow (associated with climate change or land use) alter channel (and floodplain) habitat?

3.5. Hydroecosystems in Transition

Earth’s ecosystems are in a state of transition as a result of climate change and changing land use.

The term ecosystem refers to “any area of nature that includes living organisms and nonliving substances interacting to produce an exchange of materials between the living and nonliving parts” (Odum, 1959). An ecosystem generally encompasses a region of relatively homogeneous biological

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