On the eastern side of the Central Valley, the water is snowmelt, freshly run off granite outcroppings. It is so pure and sediment-starved that it sucks the cement out of concrete, leaving the aggregate in place. This is a terrible picture, but you are looking straight down a bridge pier, with smooth concrete at the top, some erosion at the high flow line, and inch deep erosion at the water line. It is only a couple feet down to the water. The structure is about sixty years old.