Pontoon bridges are bridges that float on water. They find use primarily in two situations. First, they find application during wars when engineers need to construct a simple bridge quickly and easily. In such instances, they can be assembled from inflatable rubber or plastic and put into place in a matter of hours. Second, they can be used in rivers and lakes where the river bottom makes it very difficult or impossible to install piers. Lake Washington, in the state if Washington, for example, once had three floating bridges. All were made of large hollow concrete blocks tied to each other.