The entire Colorado Basin is eroding at about the rate Lake Powell is filling up with mud, or formerly, at the rate the Bay of California was filling up--slower than Baja is pulling away. Sandstone buildings erode less than an inch per century. Rainbow Bridge had a stream that flowed around it till the hole broke through; then the stream flowed through the hole, so a narrower gulch formed within a wider gulch. Both gulches help to give us an idea of the Bridge's erosion rates. The rocks that have broken off over the tens or hundreds of thousands of years have turned back to sand, and washed into the sea. It's a pretty slow process. --AGF