A community usually builds a dam to control the flow of a river. When a dam is built, a reservoir usually forms on the upriver side - - this is a basin that contains the water held back by the dam. Reservoirs are useful for many reasons. They can be used to store water that will be collected by municipal water providers. The water in the reservoir may be a local community's primary water supply, piping directly to a treatment plant, or it may be available as a backup supply if a primary water source becomes scarce. Dams are built to withhold massive amounts of water, releasing only as much as their operators intend. Although some communities may build dams with the primary goal of holding back floodwaters, it can be hard to predict just how much pressure a dam will be able to withstand before it breaks. It is a rare occurrence for a well-made dam to break, but it can happen - - releasing a wall of water that can level downriver communities.
A dam stretches across a river to divert or capture water flow. In contrast, a levee is built alongside the river's edges. Levees help create higher, sturdier boundaries than the river's natural banks. In this way, they serve to protect areas around a river in case the water level rises and the river floods its natural banks. However, levees have drawbacks. They may allow a river to contain more water than nature intended, but levees usually do not extend the entire length of the river. Once the excess water passes the area in which the levees are installed, it floods banks as it normally would. Being downstream from levees can be particularly dangerous during a flood, as the pressure of the flowing water will increase as it rushes from a leveed area to a natural waterway. Thus, while a levee system can protect upriver communities, it can increase danger to downriver communities.
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