A couple of years ago we were able to go on a guided hike along Onion Creek to see the karst features.
Our tour guide was Kevin Thuesen, Ph.D., Program Manager for the City of Austin Water Quality Protection Lands. The Onion Creek WQPL is located in central Hays County and was purchased by the City of Austin to help protect the Barton Springs recharge zone. The property is being managed to restore it to a grassland which will improve the quantity and quality of the water that makes it into Barton Springs.The hike was in November of 2009 and the creek was dry in many places. This provided an excellent opportunity to see the karst features up close. Karst is a geological feature, basically a hole or cave, that allows rain water to enter directly into an aquifer. They are formed by water percolating through limestone and are widespread throughout our part of Texas. There are several right in the Onion Creek bed and many scattered throughout the WQPL property. They're an important resource for recharging Bartons Springs and the City of Austin is mapping them. They're also monitored regularly. A die is injected in one and then the length of time it takes for the dye to reach Barton Springs is measured. The duration is typically on the order of a few days.
Here's Kevin discussing a large karst. You can see that it's in the middle of the creek bed. The grate was installed to try to keep too much debris from entering. The grate has to be cleaned regularly and sometimes it's under water, but that is easier than cleaning debris out from the karst itself.
This photo was taken in March, 2010 and the spring rains have filled the creek. The grate is about 5 feet under water. Although it's hard to tell from the photo, the water is actually forming a whirlpool as it enters the karst, kind of like water draining out of a bathtub. The leaf debris you see on the surface is swirling around.
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