3.4.4 Sediment filling
The lifetime of reservoirs can vary greatly. Many reservoirs have lasted for over a hundred years, but some may be useful for only a much shorter period—fifty years or so — not because of the general deterioration of the dam as it gets older, but because sediment accumulates in the reservoir. Rivers carry large amounts of mud, silt and sand in suspension, particularly during floods, and when a river enters a reservoir it slows down and the sediment carried in suspension is deposited on the floor of the reservoir. Lake Mead, on the Colorado River, has had its storage capacity reduced by over a half since the dam was completed in 1935. This is less of a problem for UK reservoirs, as rivers here are smaller and carry much less sediment. The Derwent Valley reservoirs in Derbyshire have had their volumes reduced by less than 1% through sedimentation in the 70 years since they were completed. Some water-supply reservoirs are constructed so that sediment-laden floodwaters can bypass the reservoir, but obviously this is not possible where the reservoirs are intended for flood control.