Flint Ridge Cave

Last year I got to work on a Hays County Master Naturalist project at a special place in South Austin, Flint Ridge Cave.

Flint Ridge Cave is one of the largest known cave systems in Travis County. It is also one of the most important recharge features in the county. It drains a 70 acre internal drainage basin which means that the cave entrance is the bottom of a 70 acre bowl. This is different than the typical watershed where small streams flow into rivers which eventually dump into the ocean. This watershed drains only to Flint Ridge cave which goes directly into the Edwards Aquifer. Annually, Flint Ridge cave alone contributes about 57 million gallons of water to the aquifer.

The cave entrance.

Leading our project was Matt McCaw from the Austin Water Utility, and Hays County Master Naturalist Tom Watson. The volunteers were Terry Tull, Cindy Luongo, Ruben Garza, and me. Flint Ridge is on a portion of the Water Quality Protection Lands in South Austin. The project involved planting grass seed in the area immediately surrounding the entrance to Flint Ridge Cave to stabilize and stop erosion and optimize the capture of sediments before they enter the cave itself. The grass seed consisted of several shade-loving species such as Canada Wildrye (Elymus canadensis) and Purple Top (Tridens flavus) and included the Broadleaf Woodoats (Chasmanthium latifolium) that had previously been colleted by Hays County Master Naturalists.

The city has installed three water flow monitoring chutes around the cave entrance. They measure and record the water flow as well as quality that flows into the cave.

A water flow monitoring chute.

The first step in the seeding project was to clear the leaves and debris to expose the soil.

After raking the leaves to expose the soil.

We then broadcast the seed by hand. The main goal is to make sure the seed has good soil contact and is evenly distributed.

Spreading the grass seed. Left to right: Terry Tull, Cindy Luongo.

Since the point of the project was to minimize the amount of erosion and sediments that enter the cave, we couldn't leave the soil uncovered. The next rain (this was last year when we still  got rain!) would wash our seed and some of the soil into the cave. We prevented this by spreading an erosion control mat - basically hay in a biodegradable netting. The mat protects the seeds and sprouts until they're big enough to take hold and eventually decomposes.

Preparing the erosion control mat. Left to right: Terry Tull, Matt McCaw, Cindy Luongo.

We then used landscaping stakes to hold the mat along the edges and also weighted the area with stones.

Almost done. The mat is staked down and weighted with stones. Lef to right: Cindy Luongo, Matt McCaw, Tom Watson.

As a special treat, at the end of the project, Matt McCaw, Ruben Garza, and I entered the cave and were able to explore for a short distance. I'll cover that in a future post.