Photograph of Boulder Dam on the Colorado River, showing four penstock towers under construction, March 26, 1935. Two cylindrical towers striped with thick pipes that wrap around their circumferences stand in the foreground on the left with dark walkways connecting them to each other as well as the dam on the far left. The towers undergo construction from the cranes that extend from the near canyon wall. The dam bows towards center with two more identical towers standing near the far canyon wall on the right. Automobiles drive along the top of the dam to the roadways that hug the canyon wall in the background and continue out of site on either side.