Lander, Wyoming

Popo Agie State Park

Trip Day Three: June 14, 2000
The Flow OF Water

The amount of water flowing into the Sinks varies throughout the year. Most of the time much of the limestone cavern is exposed and the sinks can handle all of the river's water. The average flow of the Popo Agie into the Sinks is about 100 cubic feet per second. this level drops somewhat during the winter and jumps dramatically during the spring run-off.

In late May and early June the river swells with melted snow from the mountains and the river fills the cavern completely. During run-off over 500 cubic feet per second of water flows into the cavern. the water is so high that logs and driftwood are often jammed into cracks in the ceiling of the cave, high above where people can stand when the water is low. The Sinks cannot handle all this extra water and the excess spills into a seasonal stream bed (called the "overflow channel"). Depending upon how much snow is in the mountains the Sinks can overflow from a few days to a few weeks. Every year during high water there is water flowing both below and above ground between the Sinks and the Rise.

What's Really Happening Here?

"The Sinks" are a series of cracks and crevices at the back of the cavern illustrates here. The water of the Popo Agie River flows into the Sinks. It then reappears at the "Rise of the Sinks", a large calm pool ¼ mile down canyon.

The Sinks are eroded into the soft white Madison Limestone formation. No spelunkers have explored very far into the Sinks since the cracks narrow down to very small log and rock choked passages.

It is unknown exactly how old the Sinks are although they are likely an Ice-Age feature thousands of years old. The glaciers that carved the canyon exposed the soluble limestone and the billions of gallons of water from the melting ice helped create the underground passages.

For many years it was unknown whether the water flowing into the Sinks was the same water flowing out at the Rise. Dye tests have proven the connection, but have also revealed another mystery: it takes the water flowing into the Sinks over 2 hours to reappear at the Rise. It was also discovered that more water flows out at the Rise than goes in at the Sinks. Why the water takes over two hours to make the journey and where it goes during that time is still a mystery.

The Geology

Text of the plaque above:
Sinks Canyon offers an excellent opportunity to study geology. The Canyon, shaped by ice-age glaciers, cuts through 400 million years of geological history. Some stratigraphy is visible above the Rise.

The uppermost formation, recognized by its light color and massive cliffs is the Tensleep Sandstone. The black and red streaks on the face of the cliffs are oxides of iron and manganese and are called "desert varnish." The next formation is the Amsden Formation, but is difficult to see because it has eroded to a grass and tree-covered slope. Below this is Madison Limestone. Through this soft formation the Sinks and Rise occur. water enters through fractures ¼ mile upstream at the Sinks, then moves laterally down through the formation. These geologic features create the unique wildlife habitats found at Sinks Canyon.

Limestone can be dissolved by carbonic acid, a weak, naturally-forming acid created by the combination of water and carbon dioxide. This acid dissolves the limestone and continued water movement will create channels in the stone. In this fashion the Sinks were created. The water reappears here through small fractures around the sides of the pool and in the sandy bottom.

Page last updated July 22, 2000.