Hydroelectric dam. The dam of a hydroelectric power station. Electricity generated by hydro electric stations is relatively cheap and involves little impact on the environment. The formation of the holding lake behind the dam, however, may have an impact on the local ecosystem. In addition to the cost benefit, hydroelectric power may be generated at a moments notice, simply requiring that flood gates be raised. As such, hydroelectric power stations are commonly used during peak demand times. This is Hoover Dam on the Colorado River. It is the highest concrete dam in the USA and impounds Lake Meade. Its hydroelectric power capacity is 1300 megawatts.