Why Are There So Many Rivers In Austin?

Austin is a paradise for swimmers and water-loving pups alike. Swimming holes are everywhere and one may as well kayak to work thanks to the network of creeks, streams and rivers criss-crossing the city. 

But in a state known for being dry and mostly flat, how come Austin is full of rolling hills, flowing rivers and pools of fresh water?

Well, it all has to do with ancient Texas and how the land beneath us formed. With underwater volcanoes, shallow oceans, faraway earthquakes and even the presence of fossil fuels playing a part, here’s the full story of how millions of years’ worth of ancient geological processes allows us to swim and splash to our heart’s content here in Austin today.

The Texas Hill Country’s rivers & how they were formed

The Western Interior Seaway 

Let’s go back in time, shall we?

The Western Interior Seaway was a shallow sea that covered much of the central and eastern United States during the late Cretaceous period (100.5 – 66 million years ago). It was created right after the supercontinent Pangea began to drift apart and the rising global sea levels allowed ocean water to flood the continent. 

This interior sea stretched from the newly-formed Atlantic Ocean in the south to the frigid Arctic Ocean in the north, and it completely submerged present-day Texas, Louisiana, Florida, Colorado, Wyoming, Montana, the Dakotas and Nebraska, as well as most of eastern Mexico and New Mexico.

The nutrient-rich seaway created a diverse ecosystem with marine reptiles, fish, sharks, dinosaurs and other animals living in the sea and on the islands that popped up in the shallow waters. The sea also supported a rich community of tiny invertebrates, plankton and single-celled organisms that thrived in the warm waters above the continent.

Though it existed millions of years before humans stepped foot in North America, the Western Interior Seaway played a major role in shaping the landscape of present-day Texas. As the tiny creatures in the sea died, their mineral-rich exoskeletons sank to the seafloor and created limestone deposits across the central North American continent. The limestone, in turn, is a huge part of the reason why there are so many rivers in Austin!

(It’s also why there is so much oil in Texas, because oil and coal were created from the carbon-rich remains of these creatures — hence, “fossil” fuels.)

The Balcones Fault Zone forms

So, now we have a landscape covered by chalky limestone. What then?

Well, after the Western Interior Seaway drained and the land dried out, the landscape that remained in the central Texas region was fairly flat and looked nothing like how the Hill Country looks today. 

Until about 20 million years ago when a huge portion of the land just… fell!

Okay, so it didn’t just fall down, but it certainly sank. 

Thanks to some subsidence (sinking) in the Texas Coastal Plain to the east, a fault zone of weak bedrock formed across Texas, running from modern-day north central Texas near Dallas to San Antonio and out to Del Rio in the west. This is known as the Balcones Fault Zone, and activity in this fault zone continued for at least 5 million years. 

To the east of this fault zone, the land dropped down by as much as 1,000 feet in places, while to the west the land stayed relatively stable. This created an escarpment that consisted of hills, valleys, rugged terrain and rocky outcroppings sloping down toward the coast. 

Today, we know this area as the Texas Hill Country, and it’s yet another important piece in the puzzle of Austin’s present-day river systems.

Erosion shapes the landscape

The fault and the resulting escarpment directed rainwater and underground water systems downhill toward the low-lying areas to the east. The steep terrain accelerated the water and gouged deep valleys into the ground, whisking away soft sediments and eroding complex waterways into the Earth. 

As erosion continued to occur, the ancient limestone bedrock began to surface to the east of the Balcones Fault. You can still see these ancient limestone shelves in places like McKinney Falls State Park, where water continues to erode and shape the old deposits. The rainwater and underground rivers also created caves all over the Texas Hill Country, thanks to the limestone’s porous nature and its fast rate of dissolution. 

Underground water shot up as it encountered cracks in the fault, creating dozens of freshwater springs that pumped water to the surface. Barton Springs Pool is one of these springs that still exists today!

Over time, the quickly flowing water from the uplands as well as the movement of water in the aquifers below created a complex series of rivers, creeks and streams all over the fault. Water kept pouring out from the fault, and the rivers and streams increased in number. Eventually, these streams fed into creeks and those creeks fed into rivers, all coalescing into the major waterways that now exist in modern-day Austin, Texas. 

A few that you may recognize are Barton Creek and Bull Creek, both of which are tributaries of the Colorado River and bring over 30 tributaries from Austin’s hilly region down to the east. These creeks are famous for their waterfalls, swimming holes and limestone caves that continue to grow as erosion occurs. 

The Colorado River is the final destination for nearly all of the rivers in Austin, taking them all to the Gulf of Mexico where their watery journeys end. At 906 miles long it is also the state's longest river, and the more than one hundred tributaries that feed it come from as far away as southeast New Mexico!

Now you know 

So, now you know why there are so many great rivers, lakes, caves, swimming holes and waterfalls in Austin. Next time you visit one of these many watery features near our Austin apartments, keep an eye out for that ancient limestone, or look for the stepped slopes along the Balcones Fault. Sure these things happened way before our species existed, but these ancient processes are why Austin exists at all — and we are certainly glad for that!

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Featured photo courtesy Pixabay/kBaucherel