It takes a dam to build a dam! No one can build anything without somehow getting the river out of the way. Engineers might build a temporary dam, called a cofferdam, upstream to divert the river into spillways that bypass the work site. Or they might construct a cofferdam around the dam site and pump water out from within it so that builders can work on the dry riverbed. Cofferdams, however, are no replacement for a finished dam—they’re only tall and strong enough to divert a river, not to create and contain an enormous reservoir.