Introduction: Featured Collection on Instream Flows—Recent Advances and the Road Ahead 1

Fish need water. We hominids have understood this basic fact since our very first interactions with Pisces. After all, of what use are fins and gills without water? Many of our fellow human beings find it incomprehensible that a river would be allowed to dry up from overuse or that a government would need to explicitly allocate water to a river. It is as though drying a river violates some universal law, or at least the laws of nature. How can a river be a river without water? Perhaps, the fact that we have assumed the existence of some universal protection for rivers explains why it has taken so long for governments to provide legal requirements for water to be left in rivers. Or maybe we just could not imagine that we could use so much water that we could cause even once-great rivers like the Colorado, the Rio Grande, the Indus, or the Yellow to go dry. Concerns for the health of rivers and fisheries in the United States (U.S.) date back to at least the 1600s, when hundreds of small mill dams were constructed in northeastern U.S. states, blocking the migrations of Atlantic salmon and other species. But concerns over the ecological impacts of water withdrawals did not begin to surface until the middle of the 20th Century, when the construction of large dams made it possible to divert large quantities of water or to rearrange natural patterns of river flow by storing and controlling water flows for hydropower generation and other purposes. In the 1950s, Donald Tennant, a fisheries biologist working for the U.S. Fish & Wildlife Service had grown concerned about what was happening to the rivers and streams he was studying. At the time, the U.S. was on a dam-building binge, constructing nearly 200 dams each year. Donald Tennant knew that these dams and the growing diversion of water from rivers and streams posed a grave threat to aquatic life. In the journal Fisheries, he wrote Philosophically, it is a crime against nature to rob a stream of that last portion of water so vital to the life forms of the aquatic environment that developed there over eons of time. (Tennant, 1976) In river basins around the world, human demands for water continue to grow, and those demands are bumping up against the limits of supply. The U.S. General Accounting Office is projecting that as many as 36 states could face water shortages by the year 2013 (GAO, 2008). The International Panel on Climate Change, a United Nations network of scientists, estimates that by 2050 up to two billion people worldwide could be facing major water shortages (Bates et al., 2008). As the world’s rivers have been increasingly tapped for water and harnessed for energy, we have come to realize that the ramifications of depleted rivers extend far beyond fish and other aquatic biota – there are