The 1950s and 60s were about the apex of dam building in the United States, and the capstone of the era was to be turning all of the Colorado through Grand Canyon into a staircase of reservoirs. The bottom and top had already been drowned in the form of Lake Mead (created by Hoover Dam in 1935) and Lake Powell (created by Glen Canyon Dam in 1963) respectively—the two largest reservoirs in the country. Two more dams were planned for the heart of the canyon: one at Marble Canyon and one at Bridge Canyon.
These dams got quite close to being built; I've explored the test tunnels drilled in the rock walls at the Marble Canyon site.
The Sierra Club led the campaign against these projects, culminating in one of the most famous advertisements of all time, a full-page New York Times spread asking "Should we also flood the Sistine Chapel so tourists can float nearer the ceiling?" In response, pro-dam forces got the IRS to suspend the Sierra Club's 501(c)(3) tax-exempt status. Though this did cause damage to their finances, membership more than doubled in protest and eventually, the dam plans were put down permanently.
Here's the full story, excerpted from one of my favorite documentaries, River Runners of the Grand Canyon:
Preserving our greatest natural wonder more than justifies keeping dams out of Grand Canyon for me, but in hindsight this was a stroke of luck even on raw economic development terms. Though clean hydropower is a nice benefit, it has been clear for twenty years and more that the truly key resource in the Southwest is water, and the Colorado is already badly over-dammed. Reservoirs are of no use if there is no excess water to fill them, and the region already consumes the entire output of the Colorado, which almost never makes it to its old outlet in the Gulf of California.
In fact, it's worse than that. Reservoirs lose a great deal to evaporation, especially in the arid and hot Southwest. Lake Powell is doubly bad, due to the porous sandstone in which it is situated, losing between ~400,000 and 800,000 acre-feet of water (130-260 billion gallons) to evaporation and seepage every year. With that and the perma-drought that has settled over the region, Lake Powell has not been full since 1999 and quite possibly never will be again.