Prior to the building of such grand projects as the Hoover Dam, the Colorado River was a truly typical major river in very many ways.
It was characterized by seasonal flooding, especially in spring. Yet for much of its journey from the snowpacks in the Rocky Mountains to the Sea of Cortez, the Colorado River cannot flood, as floods in the past have carved deep canyons which more than sufficed to contain the flood flows.
The amount of water that is delivered by the Colorado is absolutely staggering to the imagination, especially at flood, and early settlers often remarked at the contrast between the awesome aridity of the desert lands surrounding the top of the Grand Canyon and the roiling flood thousands of feet below. Any of them must have thought "if only there were some way to get that water up here, the desert could be made to bloom".
Eventually, people did manage to bring the water up to the top, not with pumps, but with megascale engineering projects. A surprisingly large number of extremely large dams impound the flow of the Colorado, and the Colorado River Compact of 1922 regulates the allocation of diversions from the flow. The total flow allocated for the upper basin in 1928 was some 7.5 million acre·ft/year, with an equal amount reserved for uses in the lower basin. California alone is allocated some 4.40 million acre·ft/year. A 1944 agreement allocates a final 1.5 million acre ft/year to Mexico.
For some decades now, it's a very unusual year when any of the Colorado River water actually makes it to the Sea of Cortez.
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In recent years, the compact has become the focus of even sharper criticism, in the wake of a protracted decrease in rainfall in the region. Specifically, the amount of water allocated was based on an expectation that the river's average flow was 16.4 million acre feet per year (641 m³/s). Subsequent tree ring studies, however, have concluded that the long-term average water flow of the Colorado is significantly less. Estimates have included 13.2 million acre feet per year (516 m³/s)[3], 13.5 million acre feet per year (528 m3/s)[4], and 14.3 million acre feet per year (559 m3/s)[5]. Many analysts have concluded that the compact was negotiated in a period of abnormally high rainfall, and that the recent drought in the region is in fact a return to historically typical patterns. The decrease in rainfall has led to widespread dropping of reservoir levels in the region, in particular at Lake Powell, created by the Glen Canyon Dam in 1963 [...] ("Colorado River Compact", Wikipedia, downloaded 2010 January 3)
Refs:
3. H. G. Hidalgo, T. C. Piechota, and J. A. Dracup, 2000: Alternative principal components regression procedures for dentrohydrological reconstructions, Water Resources Research v. 36, p. 3241-3249
4. C. W. Stockton and G. C. Jacoby, 1976: Long-term surface-water supply and streamflow trends in the Upper Colorado River Basin. Lake Powell Res. Proj. Bulletin no 18, National Science Foundation, Arlington, VA
5. C. A. Woodhouse, S. T. Gray, and D. M. Meko, 2005: Updated streamflow reconstructions for the Upper Colorado River Basin, Water Resources Research v. 42, W05415, doi:10.1029/2005/WR004455, 2006
Cycles of variations in precipitation are certainly not unknown in the Southwestern USA; it's widely believed that a native civilization known as the Ancient Puebloans or Anasazi migrated into the area during a cycle of consistent snowfalls which raised the water table of the plateaus they settled. As long as the water table remained high enough, their fortified and inaccessible Cliff Dwellings kept them relatively safe from invaders and allowed them to engage in gainful commerce with neighboring populations. The Cliff Dwellings all shared the feature of being built into cliffside caves that had "permanent" springs fed by the water tables of the mesas they farmed and hunted.
Yet eventually the precipitation patterns changed, and the Anasazi were forced to abandon their safe cliffside fastnesses when the water table no longer fed the springs in the caves.
And once the Anasazi were forced, by thirst, out of their impregnable fortresses in the cliffs, cannibalistic invaders from Mexico ate them, or, arguably, as their civilization collapsed along with other nearby civilizations (Hohokum Civano phase collapse), they were driven to eating each other.
In either case, as a culture, they no longer exist.
The Yangtze River of China, another of the greatest rivers of the world, is reported to have occasionally run dry. This was reported in the coastal Jiangsu province in 1342, and also in 1954.
As with the American Colorado river, the Yangtze is beset with flooding, though the flood patterns are somewhat different, with floods originating at times in the upper reaches, which are mountainous and fed by Glacier melt as well as direct precipitation, and at times the floods originate in the lower reaches, and at times it seems that the entire river floods from source to gulf.
The completion of the immense Three Gorges Dam will certainly do a great deal to regulate the downstream damage of floods originating in the upper reaches, and in the mere two years since the installation of the final hydroelectric generating turbine, the dam has repaid a full one-third of the total project cost simply through electrical power generation, an amazing 348.4 terawatt-hours by September 2009.
Yet it's worth pointing out that this immense investment can't always pay off at this rate, simply because the Himalayan Glaciers are melting, and that's where the upper reaches of the Yangtze gets most of its water.
The "Water Tower of Asia", composed of over 15,000 Himalayan glaciers, has been melting into the nine rivers forever. However, the rate at which they are melting today indicates that they will dry up.
The United Nation's Intergovernmental Panel on Climate Change declares that "Glaciers in the Himalaya are receding faster than in any other part of the world" and that "If the present rate continues, the likelihood of them disappearing by the year 2035 and perhaps sooner is very high if the Earth keeps warming at the current rate".
It must be noted in passing that the IPCC's declaration may be a bit premature, and at least is criticized by many as being "not supportable by the state of existing science", but then again, it's always best to be prepared for the worst and then be pleasantly surprise when the worst does not occur.
Otherwise, like most of the USA, you could be blithely drifting into the future with a defense posture and strategic position best compared to being a beauty-queen on fertility pills sleeping drunk in public with no clothes on.
Some scientific thought does indeed support the notion that Himalayan glacial melt is accelerating, due to soot particulates increasing solar absorption of glacier faces.
Other interpretations of science declare that in any case, rivers such as the Ganges get only about 5-percent of their annual flow from glacial melt, and that the rest comes from the periodic inundations by the Monsoon storms. This last may be true... but it may also be more the case that around half of the dry season flows are contributed by glaciers. In any case, the Monsoon typically brings massive flooding to these rivers, so perhaps declaring the glacial contribution to the annual flow may be technically accurate, but very misleading. Regardless of that, with sufficient reservoir capacity to moderate the differences between drought and flood seasons, a continuous release might be made to support agriculture, as is done in the US Colorado basin.
Of course, the problem with such managed systems is that you have to have something to manage. See above where we detail critique about whether or not the allocations of Colorado River water were calculated in years of unusually high precipitation.
In the worst imaginable case in the US -- not really likely but actually possible -- no precipitation falls in the Rocky Mountains, all extant snow melts, and eventually all of the water in the reservoirs is pumped to the destination. Every city along the path of the Colorado River either brings water in by truck, or the cities are abandoned to thirst. Even Los Angeles will have to be evacuated.
This would be putting nearly 50 millions on the move, looking for someplace where the water supply is reliable.
In the worst imaginable case in Asia, the glaciers melt and the monsoons fail and the reservoirs are exhausted, and about 1.5 billions on "the China side of Asia" would be uprooted, along with about 1.5 billions on the India/Pakistan side.
That's 3-billion people. That's half the world's population.
Even if you used every last known drop of oil to fuel every ship ever built, it's not possible to evacuate all of them to someplace with a reliable water supply, such as the Mid-Atlantic region of the United States.
That doesn't mean that nobody's going to try.
