The River Styx is actually the underground segment of Cave Creek as it flows through Oregon Caves National Monument and Preserve. This designation is unique within the National Wild and Scenic Rivers System as it is the first—and only—underground river in the National System.

KS Wild worked with local businesses to advance protections for the Oregon Caves and River Styx. Congress eventually passed into law the Wild and Scenic River status for River Styx in 2014. 

Managing Agency:
National Park Service, Oregon Caves National Monument

Designated Reach:
December 19, 2014. The subterranean segment of Cave Creek flowing within Oregon Caves National Monument.

Classification/Mileage:
Scenic — 0.4 miles; Total — 0.4 miles.

Thanks to rivers.gov for this information.