Rivers constantly change and rearrange their beds on their own, depending on flow and available sediment. Bars appear to move downstream over time, with new ones forming behind them. Where people come in to the process is with dams. Dams change river flow, taking out the possibility of annual floods (when you'll get a whole lot of sediment washing downstream) and generally trapping sediment in the lakes that the dam forms. Since the rivers downstream of the lakes suddenly have a lot less sediment to carry, this often leads to them eating their own bars and then not having the sediment input near the dam to build new ones.
Which is what is happening in the Grand Canyon1. We actually talked about these studies a lot in the geomorphology class that I took since it was a very good example of how sediment load affects river morphology. The Colorado River is starved for sediment below the Glen Canyon Dam, so it's been eroding its own sandbars away.
One of the proposed solutions to stop the sandbars from disappearing entirely has been changing the flow of water from the Glen Canyon Dam so that there are periodic floods, which will allow sediment to be swept out in to the river. This isn't exactly a simple proposal, though.
Ted Melis, deputy chief of the USGS' Grand Canyon Monitoring and Research Center, said Tuesday that the key to maintaining the sandbars is not simply manipulating the flows from the dam on the Arizona-Utah line. The frequency and timing of the flows would have to exceed the erosion that occurs between them, he said.
Exactly. It all boils down to how much sediment comes in to the system as opposed to how much sediment leaves. If they want to preserve what currently exists, they've got to do enough flooding and time it appropriately so that the system remains balanced. If they want to actually rebuild some of the eroded bars, they need to up the sediment input even more. It makes sense that the greatest benefit would come from controlled floods that coincide with the flooding of tributary rivers; that's when the Colorado would naturally be getting its influx of sediment.
I suppose that this can be seen as a depressing lesson that even "green" power (the Glen Canyon Dam generates electricity) isn't without a potential for high environmental cost. It sounds like a solution where seasonal flooding would make up for the presence of the dam may be workable, but would lower its power generated somewhat. To me, that still sounds like a real have your cake and eat it too kind of solution, though - the Colorado River gets its original character back, and the Glen Canyon Dam still gets to be a working dam.
Since the 1960s, Glen Canyon Dam has blocked 90 percent of sediment from the Colorado from flowing downstream, turning the once muddy and warm river into a cool, clear environment that helped speed the spread of extinction of fish species and pushed others near the edge.
90 percent? Ouch.
1 - You can tell the man in the picture is a geologist because he has a giant beard. True fact.