November 11, 2011
When a river is dammed, the water that collects in the lake above the dam stops moving, and the sediment that was carried in the flowing stream sinks to the bottom of the lake.
After some years, when enough sediment has settled, the size of the reservoir for holding water shrinks, and it becomes more difficult to manage the flow of water into irrigation systems or the turbine systems that generate electricity.
It may be possible to raise the dam, to make it higher. This will make a different lake in the same place. The Roseires Dam has been reconstructed to make it higher.
In Egypt, the dam at Aswan was rebuilt a few miles upriver, to create a Lake Nassar, because the dam in original location couldn't be built any higher.
Of course, other factors beside sediment collection have to be considered. These may include environmental issues, cost of operation, the need for more electricity, etc.
Water released from lake
Below is an amazing time lapse video showing the first stage of deconstructing a dam. It was used by an electrical company. Federal law demanded that certain environmental features should be built, and the company didn't want to pay those costs, which were going to be higher than just abandoning the dam.
A tunnel was cut into the dam at the bottom, and as the water is released in a rush, the sediment behind the dam is washed out, creating a new river bed. The dam later will be cut apart and removed.
Watch the video made in October 2011 and discover how much of the lake wasn't very deep. It was about half full of sediment. Fascinating.
Explosive Breach of Condit Dam from Andy Maser on Vimeo.
Spate Irrigation Problems
Many farmers who work in the regions around Kassala use spate irrigation. This involves building a small dam [about 2 feet high] that is very long, in a "u" shape. The dam essentially surrounds a field on the low side of flat land. This catches the water, which soaks into the land, charging it like a sponge, so that the land is moist. This allows a crop to be planted and grow even after the rainy season is over.
When it rains the water can't run off, and water from land uphill from the field will flow into the dammed area. This uphill water will bring soil with it, which will settle in the standing water. After a few seasons, enough sediment may collect in the field so that it is no longer useful because water will no longer make a "lake" above the dam.