How Dams work?

Past new year's eve, we planned to have a blast at Vegas, It was an awesome night, the strip was full of people the mood was upbeat and the fireworks were just breathtaking .

Next morning we visited Hoover dam, just a 40 minutes drive from Vegas. As we were looking at this huge dam in the Black Canyon of the Colorado River, on the border between the US states of Arizona and Nevada, there popped up question from my wife, a CPA by profession and thus less of a Techie, that how and what are these dams used for. I was so excited to answer her and explain her with the subject the "dam" right in front to point her to what I am exactly talking about.

So I started, what dam is? it is a wall of solid material built across a river to block the flow of the river to store water, the dam should be watertight and it should be stable i.e. the dam wall must have sufficient strength to firstly, stand permanently under its own weight when walls are saturated with water and secondly, resist the water pressure in the lake upstream of the dam. The next important part of the dam should be the water exit point, a dam must release water in controlled amounts through an outlet valve of some type. Depending on the purpose of the dam the water may be released into a pipeline for water supply to a city, or into a hydro-electric power station to generate electricity or the water may be released into the river bed downstream of the dam and allowed to flow naturally down the river, which can be used for irrigation of crops further downstream.

The last important part of the dam I explained was the spillway, when the river on which a dam is built floods, dam should be able to manage the large amount of water, which is usually taken care by having open cut channel large enough to carry the flood water around the dam. The spillway may form part of the dam wall itself.