That’s dam impressive construction.
A few years ago dam people decided that because we are on a fault line the earthen dam might fail flooding Columbia. So the built the concrete back-up dam. The road running along the earthen dam became two lanes running one way, from the bottom right to the top of the photo. Traffic coming from the other way dips between the two dams.
Every spring there is a Dam Swim. Couple hundred swimmers, my child included, swim the length of the dam. The route takes them on the far side of those towers, just a bit more than two miles. Parents follow along on a walkway on top of the earthen dam.