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By Root 828 0

as are 51 percent of the crayfish, 37 percent of the freshwater fish, and 36 percent of the amphibians.15 Conflicts between human and ecological uses of water have intensified in recent years: In California the delta smelt—a small fish that was once abundant in the Sacramento–San Joaquin Delta area—is in danger of extinction because its salt marsh habitat is disappearing and spawning adults get chewed up in the complex system of pumps used to transfer water to the state’s cities and farmland. In the same area, agricultural water use now threatens the very survival of Pacific salmon, whose populations have crashed so dramatically in recent years that fisheries have been closed and several subspecies added to the Federal Endangered Species list.16

Around the world, many river basins are considered “closed,” in that every drop of available water is already allocated and used, and as a result little or none reaches the sea. Perhaps the most famous closed basin is China’s Yellow River, which was so heavily used by the late 1990s that the river failed to reach the sea for almost the entire year.17 Water managers at one point recorded 687 kilometers of the river’s length with no flow at all. In the Middle East, so much of the River Jordan’s flow is diverted into Israel’s National Water Carrier that little if any reaches the Dead Sea, whose level is now falling precipitously.18

Dams and reservoirs tend to iron out the peaks and troughs of high-and low-season river flows, yet many species are adapted to respond to these seasonal signals. For example, the Missouri River in the United States once experienced annual snowmelt floods, when warmer spring temperatures thawed snowpack in the Rockies and released a pulse of water into the river. These floods distributed seeds and nutrients and allowed fish to move onto the floodplain to feed. In the later summer, low-water periods would expose sandbars that were crucial for nesting birds, leave large shallow areas and braided channels essential for spawning sturgeon, and expose extensive mudflats where migrating birds would forage for invertebrates. Today most of the Missouri’s flow is controlled by six large federal dams operated by the U.S. Army Corps of Engineers. As a result of the disappearance of the natural flow regime, fish can no longer access floodplains, and crucial shallow water and bankside habitat has disappeared, while the flow changes that provided critical seasonal life-cycle cues to many different species no longer appear. The ecological toll has been disastrous: Of 67 native fish species living in the river, 51 are now classed as rare, while the pallid sturgeon and two bird species (the least tern and the piping plover, both dependent on vanishing sandbanks for nest sites) languish on the federal Endangered Species List.19 Downstream from many hydroelectric dams, river heights can vary by meters in just a few minutes as vast quantities of water are released to generate electricity—leaving animals and plants alternately swept away or stranded high and dry on the riverbank.

Another key ecological water characteristic is temperature. The Colorado River in the U.S. and Mexico was changed forever by the construction of the Glen Canyon Dam in 1963, after which natural seasonal changes disappeared and water was released from 60 meters down under the newly created Lake Powell at a uniform 9°C all year round. In many rivers, thermal regimes are as important as flow regimes in providing seasonal life-cycle cues to various species, provoking fish to spawn as water temperatures rise and insects to emerge from pupa. While water extraction leaves less water in the river’s main channel—meaning the remainder tends to heat up more quickly—water released from thermally stratified large reservoirs like Lake Powell tends to be much colder than undammed flows. Large distances are needed to mitigate this thermal pollution: Estimates suggest that for the Colorado’s water to recover its natural temperature downstream of the Glen Canyon Dam would require 930 kilometers of flow—impossible anyway