... type of water power has been used for centuries... hydraulic schemes are used to generate large amount of electricity. ..., a dam is built across a river or lake. A reservoir behind the dam collects the water… electricity is needed, the water is allowed to rush through holes in the dam... hydroelectric schemes have to be carefully planned …they can cause large areas of flooding. Mỗi chỗ trống điền 1 từ: about, although, moreover, however, nowadays, since, this, that, usually, when, which