The Rise of the Sinks is a large spring where water from the middle fork of the Popo Agie River reappears after flowing into a limestone cavern called the Sinks a quarter of a mile upstream. The water moves underground along an unknown path, and geologists speculate that it travels through cracks and fissures in water-soluble limestone before rising to the surface in a calm pool. The Rise is an active geologic formation that changes from year to year as erosion in the river and underground alters water flow patterns and changes the amount of sand flowing into the pool, so that in some years the sandbar is almost gone and in others it occupies part of the upper end of the Rise. Shaped by ice-age glaciers and the power of the river, Sinks Canyon has served as a natural travel corridor for wildlife and people for thousands of years. Known to Native Americans for centuries, the Sinks and Rise were first seen by whites who were fur trappers. Popo Agie, pronounced Po-Po-zsha, is a Crow Indian word meaning Grass River. The Rise was donated to the City of Lander in 1969 by Pacific Power and Light Company, and the site is now part of Sinks Canyon State Park administered by Wyoming State Parks and Historic Sites.