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A year ago, I joined the Wimberley Outdoor Educators (WOE) team supporting the Jacobs Well Elementary School students on a field trip to the Blue Hole Regional Park. Jackie Mattice assigned me to work the water station and one topic was to explain how the Blue Hole was formed. It was then I realized that my knowledge of how the Blue Hole works was not very good. That question has stuck with me throughout this year and I have worked to better understand how the Blue Hole was formed and where the water comes from. Here is what I discovered.
The easy answer is that the water you swim in at the Blue Hole comes from Cypress Creek. Everyone knows that, but what part and what is the hole? Cypress Creek’s length is 14.4 miles, but it is dry for the 9.5-mile upper segment above Jacob’s well. Over 65% of Cypress Creek is not contributing significant surface flow down to the Blue Hole. Only during high rainfall events does the dry upper segment actually flow. The 4.9-mile creek segment below Jacobs Well has a continuous flow. Nearly all of the creek flow in this section is groundwater coming to the surface via springs. There are three major spring flows within this stretch providing water to Cypress Creek and ultimately the Blue Hole.
Jacob’s Well spring is the headwaters of Cypress Creek, flows most of the year and is a significant contributor to the Creek. Looking at the volume of water at the Blue Hole leads me to believe that the flow from Jacob’s Well is supplemented by additional springs. Take a look at the photo of Jacobs Well spring in March of this year. This was during a moderate drought and the flow was only about 1800 gallons per minute. Also note the absence of the pool. The downstream dam that creates the shallow pool at Jacobs Well was bypassed due to repairs allowing a view of the actual spring flow. The photo illustrates how the drought and increased pumping have impacted Jacobs Well. Some good news, the many months of recent rains have brought Jacobs Well flow rate up to almost 8,900 gallons per minute as of November 13.
I live about a mile from the Cypress Falls low water crossing on Woodcreek Dr. I am always in this area, meeting friends, eating some good food and looking at the great swimming hole just behind the lodge. This area is known as Eagle Rock. There are large pools above and below the low water crossing. They are a beautiful sight. Even though the pools are a result of low dams, there is a lot of water flowing here, more than what I see coming out of Jacobs Well. All of these signs point to a set of springs in this area. A quick look at the geologic map confirms that two segments of the Tom Walker fault slice though Cypress Creek right in this area. I already know that Jacob’s Well spring was created along a fracture zone which is right in line with the Eagle Rock area. These are the perfect conditions to create springs in the Wimberley Valley. The water from these springs at Eagle Rock joins with the flow from Jacob’s Well and continues down Cypress Creek to the Blue Hole.
Ever since I did my first launch off the rope swing, I have wondered if there are springs at the Blue Hole? When you enter the water, that deep cold pool feels like spring fed water, this is the “Hole” at Blue Hole. Also known as the deep end. Over the years I have been swimming at the Blue Hole, that deep pool has never filled up with rocks or sediment, even though the area has seen many big floods. I think this pool maintains its depth from a strong spring flow that removes debris deposited into it. I have seen the same thing when I take the Glass-Bottom Boat excursion onto Spring Lake in San Marcos. The boat passes above many impressive springs which can be easily observed by looking through the glass bottom. The springs are always the deepest underwater view you get. My interpretation is that the flow of the water self-cleans the area around the spring moving rocks and sediment away, creating a hole. This same process is happening at the Blue Hole. My take on the geologic setting is that the subsurface fracture zone observed at Jacob’s Well extends past Eagle Rock and intersects the Wimberley Fault at the Blue Hole. The fault allows the water moving though the fractures to flow to the surface creating the spring at the Blue Hole.
The WOE Blue Hole field trip was cancelled this year due to the very cold weather in November. Hopefully it will be held next year, and Jackie lets me work the water station again. I’ll be ready with my answer. Or maybe just the short version.