A VANISHING ACT: UNDERSTANDING THE PATH OF THE POPO AGIE RIVER THROUGH THE SINKS CANYON CAVE

Doing large-scale experiments arguably best teaches hydrogeology students about the physics of subsurface flow. We present the results of a unique, large-scale, tracer experiment done at the 2006 Branson Geology Summer Field Camp (University of Missouri) to instruct students on the physics of solute transport. The hydrogeology students dyed the entire Popo Agie River, which serves as the water source for Lander (WY), to determine how long it takes for it to reappear in a series of springs (“The Rise”) once it literally disappears into bedding plane fractures in the Madison Limestone (“The Sink”). The direct distance between the sink and rise is only about 400 meters.

The peak of the tracer, Rhodomine WT, arrived at the Rise about 2.8 hours after injection at the Sinks. The travel time of the dye approximately agreed with the results of a United State Geological Survey dye test done over 20 years ago. Both studies identified an increase in the discharge of water in the Popo Agie along the subsurface flow path and an increase in temperature of about 3 degrees Centigrade.

The breakthrough curve of the dye was asymmetrical with an extended tail probably caused by retention and dilution in the cave's complex plumbing system. It is possible that increased water flow and temperature measured at the Rise are caused by the mixing of water from a much smaller and shallower adjacent stream, Sawmill Creek, that also disappears down a sinkhole in the immediately adjacent watershed. However, the Sawmill Creek would have to have a temperature of about 25 degrees C to cause the 2.6°C increase between the Sinks and Rise. Future dye tracing studies at the Branson Field Camp will involve doing multiple dye tracing of both stream systems.