Imagine a decorative wishing well, then imagine that well writ large. Next, understand that the well is not meant to contain water but to exclude it, so you must now envision a body of water surrounding your big, dry well. These are the kinds of structures erected by engineers in the middle of a river then pumped dry; the watertight wall enclosing the dry area is known as a cofferdam. In his book Boulder Dam, Zane Grey describes one such structure this way: “The truck road ended on the cofferdam, which was still forty feet above the river—a solid obstacle of lumber and sandbags.” Built as temporary structures to facilitate the construction (or destruction) of aquatic edifices such as bridges or dams, cofferdams have been employed since the early days of modern dam building. In the early 1900s, large cofferdams were put in the great Mississippi River to hold back its water, allowing the construction of what was then the largest hydroelectric plant in the world—Keokuk Dam in Keokuk, Iowa

Antonya Nelson