The floodway, started on October 6th, 1962 and finished in March 1968, was a major undertaking with 100 million cubic yards of earth excavated - more than what was moved for the Panama Canal. The construction left such a mark on the face of the earth that it was visible to the Apollo Program on the Moon.
Please find a reputable citation for the claim that the site was visible from the moon. It's been established that the Great Wall of China, long thought to be visible from the moon, was in fact not at all visible. Some of the astronauts have said that even from low orbit, Earth looks like a blue-white-green ball, with little if any obviously man-made detail visible.