Dependent on a dwindling river, the American South-West turns to radical engineering projects to keep the taps running.
About 38 million people in seven US states and Mexico depend on the Colorado River for their water supply. As increased usage and years of drought diminish the river’s flow, states are forming strategies to deal with what some experts call peak water – the point at which the demand exceeds the supply.
The conditions already threaten Las Vegas, which draws water from the Colorado-fed Lake Mead, America’s largest reservoir. During the past decade, Mead’s level has fallen more than 30 m, and the basin is now just over half full. The Southern Nevada Water Authority draws nearly all of its supply from the lake through two intakes, one of which is just 18 m below the current water line; the second intake is 15 m below the first.
If the lake keeps shrinking, by 2012 the upper intake may suck only air. Intent on addressing the problem, local authorities have embarked on a R7,3 billion-plus project to bore a 4,8 km-long tunnel beneath Lake Mead to tap the lower reaches of the lake (see below).
Although the Colorado rose in 2008, its flow can double one year and drop by half in the next. During the past eight years, the river has carried less water than in any similar period in its recorded history. “With demands increasing, droughts are going to hit harder,” says Terry Fulp, deputy regional director of the US Bureau of Reclamation, which manages the river’s large reservoirs.
“There was a drought in the 1950s, but there weren’t as many people.” The Colorado River water crisis will linger even if flow increases; by 2030, the population of the area it serves is projected to grow to more than 65 million people.
How it’s done: Vegas water pipeline hardware
A specially designed tunnel-boring machine is being constructed in Germany to dig a water-intake pipeline under Lake Mead by 2012. Engineers planned its path to minimise wear on the cutter heads, but the machine still must slice a 6 m-wide tunnel through volcanic layers 20 to 30 times tougher than concrete. Lying beneath 106 m of water, the tunnel will be under extreme pressure during the dig. Crews will have to acclimatise in a pressure chamber before descending to replace the machines tips if they break. Workers are sinking a 180 mdeep shaft on the shore to reach the depth where horizontal tunnelling will start late next year.
Solutions for peak water
The seven states that draw water from the overtapped Colorado River sponsored a 2008 study that analysed options for supplementing the rivers flow. Here are the most intriguing:
Las Vegas could pay for desalination plants along the West Coast in exchange for the right to use Californias or Mexicos share of the Colorado.
States could build offshore aqueducts that would run under the ocean from the Columbia River, in the Pacific North-west, to the South-west.
3. Import water
Some parched parts of the world already import water via tanker ships or by towing giant water bladders across the ocean. The report considers using such methods to move water from Alaska to California and the Southwest. It also looks at towing insulation- wrapped icebergs from the Arctic.
4. Kill shrubs
Settlers in the 1930s introduced fast-spreading saltcedar plants to stabilise the banks of the Colorado River. A single mature tree can absorb as much as 750 litres of water per day.
Using less water is cheaper than developing new supplies. Methods include collecting overfl ow from storms, limiting the use of water to cool power plants, and treating agricultural and industrial wastewater to make it potable.