io9.com recently posted a nice set of pictures here, of Niagara Falls being shut off for maintenance  in 1969 by the Army Corps of Engineers.  Here’s what they looked like from Prospect Point when off:

The Falls when dry, looking from American side

And here’s what they look like when on:

Standard postcard view

The pix originally came from Rob Glasson’s Flickr Photostream.  The Corps was worried about erosion on the rock face, and also wanted to remove some talus at the bottom of the cliffs.   They closed off the flow with a dam about a half mile upstream:

Aerial view of the shut-off of Niagara

The talus apparently appeared in a rockslide in 1954.  The Corps decided not to actually remove it – too expensive, and perhaps bad for the cliff face.  They also kept the bed watered to prevent it from drying out and cracking.  The extra water went over the Horseshoe Falls on the Canadian side.

They kept if off for six months, from June to November 1969.   They clearly weren’t thinking of the tourist business, or else they would have waited until autumn.

They may have been more worried about the hydroelectric business.  At present there’s a total of 4.4 GW of generating capacity on both sides of the river.  That’s 1% of the total consumption of the United States!  At 10 cents a kWh, that represents $10 million a day.

The US part of the project is mainly at the Robert Moses Niagara Project, which is a 2.4 GW dam/pumped-storage facility in Lewiston NY, and is run by the New York Power Authority.  Their annual budget shows that it runs at about 70% capacity, producing 1.7 GW on average, totaling 15 TWh a year.   The whole NYPA had revenue of $2.8B in 2008, and a big part of it must have been from this.  They like to run their generators at night when the flow over the Falls is less important to tourists, and use the power to pump water up into a holding lake.  They then drive the generators from the lake during the day instead of from the river.  The electricity is worth more then, and the Falls flow is preserved.  Clever!  That trick is probably worth hundreds of millions a year.

Anyway, it’s nice to think that this awe-inspiring, overwhelming spectacle is now a carefully preserved artifact, a kind of Nature’s Grandeur theme park.  The Falls could be reduced to a trickle to feed the insatiable demand for cheap power, but they’ve kept it flowing out of atavistic respect.  When the day comes that people care more for artificial spectacles than natural ones, and care more about CO2 levels than the tacky hotel business, the Falls will be gone.