Hydrologic oddities: My first underground river

The first strange river I ever heard of was the Middle Fork of the Popo Agie. At Sinks Canyon State Park, an hour west of Lander Wyoming as the river flows (or 10-15 minutes as the car goes), the Popo Agie River dives into a cave and then calmly emerges a brief walk downstream. The pace of the river seems a bit like racing into a bathroom stall and then strolling out, relieved, except in this case the river returns a bit larger than when it went in.

The Sinks (source) where the river goes in

The Rise (source) where it comes out. Note swarms of fish in lower right.

Because it has an unusual pronunciation, I had read and heard the name "Popo Agie" for months before I put it all together. It's not poh poh aggie (as in rhymes with Maggie). It's pronounced poe (as in Edgar Allen--) poe-juh (as in juh, justice). Puh pojuh. Such a mixup would cause exasperated sighing and eye-rolling from the locals, as if one was pronouncing japaleno with a j (as in Japan) instead of an h.

The river travels about 10 miles (16 km) from the mountains before reaching "The Sink". It then goes underground for about 1/4 mile (400 meters) until it reaches the "The Rise". It would be the distance-to-age equivalent of a high school student's 3 month summer vacation. What a memorable summer that must have been! Also, if streamflow were a person's weight, the river would have put on the equivalent of about 25 pounds after it came back from vacation.  The stories it could tell.

The large view. The river flows from the mountains and out of the canyon onto the plains. 

The close up view. The river "sinks" in the lower left and rises again downstream in the upper right. 

Even though I had been a surface water hydrologist for about five years, the idea of a disappearing river made my mind swim. For a given sized area, drier areas had smaller rivers than wetter areas, clearly. I also pictured some rivers like the Colorado getting exhausted from overuse, limping along until dying of thirst before reaching the ocean. The image of a river willingly disappearing, though, in it's prime, seemed like hydrologic death and resurrection.

The rocks in the area (like for most strange rivers) are limestone, carved in part by glaciers. Water dissolves some of the ground as it passes through limestone, like sugar in tea. The rocks can have empty pockets underground that eventually can get big enough to form caverns. Sometimes these pockets are partly exposed to the surface and rivers can flow in.

In the case of the Popo Agie, the water collects underground and seeps through the rock until it comes to the surface in a placid pool called "The Rise". A couple weeks a year, when the flow gets very high, the cave backs up, starts to overflow and connects up with the downstream through conventional overland means like more normal rivers do. If you want to see what the river is up to right now (some ways downstream), you can see the data here, as well as many photos of the river gage itself.

Relatively little is known about what is happening to the river while it is underground, because it is impossible to go into the cave. Instead, students, as part of a field camp for geology students, put dye in the river upstream and watched it come out downstream (about two to three hours later). It was a bit like splashing paint on cars as they drive into a tunnel and timing when painted cars start coming out the other side.

It was all very different from my textbook vision of how rivers should behave. Imagine if a garden-variety surface water hydrologist was captured, hooded, and released somewhere between The Sink and The Rise without telling him that the underground river existed. If he was asked to model the river flow (this isn't how it usually happens, but let's just pretend), he would probably end up quite confused as to why there is an empty channel. If he never visited the river in person, he might guess that the gage is giving bad data.

The Popo Agie is a very visible example of something dramatic happening to a river because of what is going on underneath the surface. Although the river is the exception rather than the rule, what it made me appreciate was all the other cases in the world where less obvious but still quite quirky things are going on underground.