Potentially, one of the most dangerous effects of global warming is a decrease in river flow. According to a study published in the Journal of Climate, rivers worldwide are losing water as a result of global warming. The study, undertaken by scientists from the National Center for Atmospheric Research (NCAR), looked at river flow from 925 rivers between 1948 and 2004. They found that stream flow decreases were more than twice as common as increases. Decreases were especially common in populated areas: among the rivers losing water were the Yellow River in China, the Niger in West Africa, the Colorado in the United States, and the Ganges in India. All in all, river water flowing into the Pacific Ocean fell by 6 percent, or by as much water as flows into the ocean from the Mississippi every year.
Some of the reduction in river flow in these areas was due to dam construction and the diversion of water for irrigation. The Columbia River, for example, lost 14 percent of its flow over the 50 years of the study, partially because of increased water usage. The reduced flow also reflected a fall in the region's precipitation, however, which could be linked to climate change. Worldwide,
"The researchers found . . . that the reduced flows in many cases appear to be related to global climate change, which is altering precipitation patterns and increasing the rate of evaporation."1
Some rivers did show increased flows. The Mississippi, for example, increased by 22 percent because of greater rainfall in the Midwest. Rivers in sparsely populated areas in the Arctic, where increased melting has dumped freshwater into rivers, also increased their flows. So did the Yangtze River, though scientists worry that this river may be threatened in the future as the Himalayan glaciers that feed it melt away.
Falling river flows are worrisome not only because of the dangers to human water supplies, but also because freshwater has an important impact on the oceans. Rivers deposit nutrients and minerals into the oceans, and these nutrients are vital for many sea creatures. In addition, river discharge affects ocean circulation and currents. So far, the changes in river flow have not been large enough to cause major changes in ocean patterns, but researcher Aiguo Dai has noted that "freshwater balance in the global oceans needs to be monitored for any long-term changes."2
Global climate change may affect river water in other ways. As mentioned in Chapter 3, when sea levels rise, salt water moves further upstream in rivers. This encroachment can contaminate some water supplies, especially in such places as New York City, Philadelphia, and California's Central Valley, which have water intakes only "slightly upstream from the point where water is salty during droughts."3 Similarly, high evaporation rates and changing precipitation patterns can result in contaminated freshwater supplies. In the U.S. drought in the summer of 1999, for example, there was not enough freshwater to cleanse rivers and streams; as a result, "salt water encroached further up rivers in many areas of the mid-Atlantic coast."4
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