In Sinks Canyon State Park in west-central Wyoming, the Middle Popo Agie River does something few rivers do: it disappears underground. At a point known as the Sinks, the river's entire flow vanishes into a cave system instead of continuing along an open channel. Downstream, the water reemerges at a separate location known as the Rise, where springs return the river to the surface. Although the underground relationship between the two points had long been recognised, U.S. Geological Survey hydrologists wanted to establish the connection through a direct test. In August 1983, they released fluorescent dye into the Middle Popo Agie at the Sinks and monitored water downstream at the Rise for the tracer's return. The dye appeared at the Rise about two hours later, providing direct evidence that water entering the Sinks travels through the underground system before resurfacing at the Rise. Researchers recovered about 84 per cent of the injected dye during the investigation, providing further evidence of the connection.
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