Big Gav demonstrates the effects of playing with nature in the southern hemisphere while Phila does the same service courtesy of the north.
In both cases, altering the environment leads to unintended consequences. In the southern hemisphere, deforestation along the Amazon River has likely exacerbated the effects of the current drought, causing much reduced flow along the river and in particular, its tributaries.
In the northern hemisphere, you first have to understand the history of direct manipulation of the flow of water by humans. Read Cadillac Desert by Marc Reisner for a good background. The developers considering redeveloping along the Salton Sea might want to pause and take a deep breath before getting their building permits.
In 1905, floodwaters caused a levee to break on the Colorado River near Yuma; water rushed into the Salton Basin. This created the Salton Sea, about 68 feet deep, 55 miles long, and 16 miles wide, with a total water area of some 300 sq. miles. Since the break threatened the agriculturally rich Imperial Valley and a major railroad route, the levee was finally repaired in 1907, but the Salton Sea remains.Interestingly, the breech of the levee, actually an irrigation diversion structure, has some similarities to what happened to New Orleans. Fortunately, a teeming metropolis didn't get built up in that area.
Commerce & Politics
The Colorado is a remarkable source for hydroelectric power and irrigation. Of its 10 million potential horsepower, one-fifth has been developed. 21 dams have been built on the Colorado and its tributaries. The river rarely reaches the Gulf of California because of these dams. The Morelos Diversion Dam, located on the Mexico-Arizona border is the southernmost dam on the Colorado. It sends virtually all of the remaining water to irrigation canals in the Mexicali Valley and to the towns of Mexicali and Tijuana.
If the Amazon basin turns into a desert, expect a similar course of events. Humans will dam the river to water the crops, weird crap happens, a sequel to Chinatown gets made, etc. I can't imagine the mightiest river in the world reduced to a trickle before entering the ocean, but history shows it could happen.
I didn't mean to meander so much in articulating my thoughts in this post, but a good flow of water will do that -- gravity makes it so.