One of the big reasons the dam was built to control floods. It protects large cities like Wuhan, Nanjing, and Shanghai, which are very close to the river. Lots of farms, small villages and towns, and factories are also located near the river. In the event of a flood, there are 22 cubic kilometers (18 million acre feet) of space behind the dam to hold back the flood waters. The flood waters are stored behind the dam for future use. In the dry season, the water is released down the river, and the reservoir level drops to prepare for the next flood. However, the dam does not protect the lower river tributaries from flooding.