How long a dam lasts and how long the reservoir behind it is usable are two different things. A well constructed dam may last 100+ years but if the water flowing into the lake carries heavy sediment loads, the lake will fill up with mud before then if not periodically drained and the muck removed. This is definitely an issue with Three Gorges as well as Lake Powell&#x2F;Glen Canyon Dam on the Colorado River. In fact, one of the reasons the latter dam was constructed was to act as a catch basin so the downstream Lake Mead behind Hoover dam wouldn&#x27;t fill so quickly.