After the devastating flood of March 1938, it became clear that the Los Angeles area needed a stronger network of flood control features, and fast. The U.S. Army Corps of Engineers led a remarkable effort over the next few years to channelize the city's rivers, streams, and major washes, crowning it all with the construction of Hansen Dam. Located in Big Tujunga Wash in the northeastern part of the San Fernando Valley, at the time of its construction the dam was the largest of its type in the world. It was designed by a large team of engineers in the U.S. Engineering Department and built by a workforce of nearly 1,000 men, supervised by general contractor Guy F. Atkinson Co. of San Francisco.

Hansen Dam is an enormous horseshoe-shaped earthen wall, up to a quarter mile wide at its base, with a monumental concrete spillway and outlet system at its center. The Moderne structure is designed not to stop the flow of water, but to slow and redistribute it to prevent catastrophic flooding events. The concrete spillway is topped by a long bridge supported by slender, wide flanges of sculpted concrete. Below it, gates and the main channel regulate the flow of water into an ever-widening area. A simple, hipped-roof concrete tower stands guard over it all, sheltering equipment and dam personnel monitoring all activity. The dam is a stunning illustration of the functional and aesthetic power of good design and construction, as epitomized by government work at the end of the Great Depression.