Hoover Dam and Lake Mead: a strange coincidence
Hoover Dam between Arizona and Nevada was a landmark in civil engineering. At its completion in 1935 it was the largest concrete structure in the world (and was the first single structure to surpass the total masonry of the Great Pyramid at Giza). It took about five years to complete and now holds back Lake Mead, the largest water reservoir in the USA and crucial for the water supply of much of the south west (about 25 million people). Its capacity is more than fifteen times that of Lake Burragorang behind Warragamba Dam in Sydney.
The Hoover Dam Bypass (a.k.a. the Mike O'Callaghan – Pat Tillman Memorial Bridge), opened to pedestrian and cycle traffic yesterday and is the longest single arch concrete bridge in the western hemisphere. Taking about five years to construct and soaring over the Hoover Dam, it makes for some impressive photos and removes the need for traffic to use the two lane road across the top of the dam wall, a slow section of a major infrastructure route and a potential target for a vehicular bomb (large trucks have been banned from the road since not long after the attacks of 2001).
On Sunday (the day before the bridge opened), Lake Mead fell to its lowest level since it was filled in the 1930s, due to a complex range of factors, most notably water management over-allocation and persistent drought. Declining precipitation in the south west USA has for some time been predicted by most climate models as a result of anthropogenic climate change.
So within a twenty four hour period, a major new bridge opened to allow more traffic (and so more carbon dioxide emissions) over a dam whose reservoir is being emptied (partially) by climate disruption resulting from those emissions. Irony?
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NYT: "Mr. Nelson said that the 11-year drought, which has caused the Colorado River to deliver considerably less water than its users have been promised, "reflects weather patterns that are what climate models predict for an era of climate change." "Either these are early indicators of climate change or conditions we should expect more of in the future," he concludes."
Wikipedia on Colorado River Delta: "Today, conditions in the delta have changed. Like other desert river deltas, such as the Nile Delta and the Indus River, the Colorado River delta has been greatly altered by human activity. Decades of dam construction and water diversions in the United States has reduced the delta to a remnant system of small wetlands and brackish mudflats.[2] As reservoirs filled behind dams and captured floodwaters, freshwater flows no longer reached the delta.
"The construction of Hoover Dam in the 1930s marked the beginning of the modern era for the Colorado River Delta. For six years, as Lake Mead filled behind the dam, virtually no freshwater reached the delta. Even spring flooding was captured. This ecologically devastating event was repeated from 1963 to 1981 as Lake Powell filled behind the Glen Canyon Dam. With these reservoirs now filled, the dams are used to regulate flow so that water can be reliably apportioned among the users of the Colorado River Compact, and its use maximized. Most flood flows can be contained, regulated, and added to the river’s capacity to sustain the Western United States' urban centers and agriculture. Floodwaters are released only when the Bureau of Reclamation, the agency managing the dams, predicts flows that exceed the system’s capacity for use and storage.
"The loss of freshwater flows to the delta over the twentieth century has reduced delta wetlands to about 5 percent of their original extent, and nonnative species have compromised the ecological health of much of what remains. Stress on ecosystems has allowed invasive plants to out compete native species along Colorado River riparian areas. Native forests of cottonwood and willow have yielded to sand and mudflats dominated by the nonnative tamarisk (also known as salt cedar), arrowweed, and iodinebush, a transformation that has decreased the habitat value of the riparian forest (Briggs and Cornelius, 1997)."
Southwest to suffer: even the best case scenario looks bad.
SW US water scarcity issue predicted 25 years ago: "On the other hand, Gleick warns, the water problems in the Southwest are bound to get worse in the coming years rather than better. 'I’m not sure we’re moving fast enough to avoid more serious disruptions in the future.'"
Running toward empty and part two. A good summary of where things currently stand (Jan 2011) and longer term challenges.
Lake Mead's winter bump.
Climate Central: Update on water in the SW. The bottom line is that the news is relatively good in the short term (some respite from the dropping water levels) but still bad in the long term.
Climate Central: Dry weather is drawing down Lake Mead.
CP: How Two Reservoirs Have Become Billboards For What Climate Change Is Doing To The American West.
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