Their rich biodiversity aside, freshwater systems bestow untold—and underappreciated—benefits on people. Indeed, they are the very foundation of our lives and economies. The value of all the services freshwater ecosystems provide worldwide, such as drinking water, irrigation for agriculture, and climate regulation, has been estimated at $70 billion per year—a figure that assumes, rather delusionally, that one could purchase the services elsewhere if they became unavailable in nature.
Dams are a dramatic example of a human activity that degrades freshwater ecosystems. Built to control flooding, store water, and generate electricity, dams have numerous ecologically disastrous side effects. They impede the movement and migration of aquatic species; some kill animals in turbines; and they change the timing and amount of flow downriver, which interferes with the reproductive cycles of fishes, frogs, and water birds that depend on seasonal flooding.
About a dozen hydroelectric dams in the Mekong River Basin provide the bulk of the region’s energy—and another hundred or so are in the planning stages. To date, China has built two dams across the upper mainstream, but there are none across the lower mainstream—in fact, the Mekong is one of the world’s few major rivers with so few mainstream dams. That may soon change: local governments view the free-flowing Mekong as an underutilized economic resource. Worldwide, an average of two large dams have gone up each day for the past fifty years, and today there are more than 45,000 dams taller than forty-five feet. Fortunately, increased awareness of the environmental problems they cause has contributed to a slowdown of large-dam construction in the United States and Europe. In the Mekong River Basin and elsewhere, however, big dams continue to rise.
Species along the Mekong, as in other freshwater systems, depend on natural flood cycles for nutrients and for transportation to and from spawning grounds. More than 90 percent of the fish species in the Mekong watershed spawn not in rivers, but in seasonal lakes or periodically flooded forests and fields. Flow patterns altered by dams and other projects could prevent those species from reproducing. In addition to building dams, countries along the Mekong are destroying or modifying rapids and other natural features to improve navigation—changes that will disturb critical fish habitats and alter downstream water flow.
Worldwide, irrigation guzzles about 70 percent of the freshwater people use. To grow food for expanding human populations, people divert rivers, drain inland seas, and extract fossil groundwater collected over thousands of years, often at unsustainable rates. Worse, current agricultural practices often waste as much water as they use: about half the water that flows through conventional irrigation systems never actually reaches a crop plant. A lesser—though still formidable—amount of water is siphoned off to slake the thirst of cities and industry, and when you add it all together, it’s clear that people are using more than their fair share. The Mekong still manages to reach the sea. But at least ten other major rivers, including the Colorado, Ganges, Jordan, Nile, Rio Grande, and Yellow, now regularly run dry before they reach their outlets.
Agriculture, in addition to being the greatest consumer of freshwater, is also a major polluter—another bane for wildlife. In the Mekong River Basin, agriculture relies heavily on pesticides and fertilizers; it also drives deforestation, which causes erosion. Chemical, nutrient, and sediment runoff from farms winds up in the Mekong River Delta, where it degrades water quality, shifts natural nutrient cycles, and alters wildlife habitat. The six nations in the Mekong watershed have initiated a regional program to encourage agricultural development. If not done mindfully, the accelerated development could worsen water quality.