This state park is appropriately named. The Middle Fork of the Popo Agie flows out of the Wind River Mountains and through the canyon. The river, halfway down the canyon, sharply turns into a limestone cavern, sinking the water into cracks in the back of the cave. From there, the river goes underground. No one knows where the water goes while underground; the sinks are unnavigable. For many years, it was not known that the water appearing out of the Rise was the same water flowing into the Sinks. Dye tests have proven that it is the same water, but interestingly, the water takes over two hours to reappear at the Rise. Also, the water flowing into the Sinks is greater in quantity than the water flowing out of the Rise. Geologists think that the water, while underground, goes through various narrow, winding passages and pools before it resurfaces. The Sinks likely date back to the Ice Age. The Indians knew of the Sinks for many generations before the first white men saw it in the 1800s.