Lost Rivers

When the bedrock of an area is comprised of limestone, interesting things happen. The Earth yields caves dripping with stalactites, pooling aquifers, sinkholes, bubbling springs, artistic features like pepino hills and natural arches. Water picks up CO2 on its descent from the clouds, making it acidic, which causes it to erode the calcium carbonate in limestone/dolomite into a pockmarked, Swiss cheese subsurface. These are called karst landscapes. 

The term was coined in reference to northeast Italy/Slovenia, a rocky region geologically similar to many locations around the world. The cenotes of the Yucatan Peninsula in Mexico, the stone trees of Shilin, China, Mammoth Cave in Kentucky and the Edwards Aquifer in South Texas, where my freshwater comes from, are all karst formations. The National Park Service says 20% of the US is a karst landscape and 40% of our groundwater for drinking is in karst aquifers. 

One of the phenomenon that happens in these areas is "lost rivers." Also known as "sinking creeks," "ghost rivers," and "disappeared streams," this is where a flowing current goes underground. The water descends into a swallet (as opposed to a sinkhole, which is any depression, a swallet is where water enters an underground system) before reemerging to the surface further downstream. 

These submerged rivers and labyrinthine caves are described by John Jeremiah Sullivan in Pulphead, an essay collection on the South from 2011. He mentioned how in the Cumberland Plateau, people had witnessed ancient tributaries cease in an instant, the limestone river bed suddenly collapsing upstream and sending the rush below. Any rational mind would see it as an act of God. I couldn't find any evidence of this actually happening so immediately besides his essay; I did find a story about a stream in Arlington, VA disappearing into a sewer pipe after a storm, though. 

The Texas karst landscape provides not only ample drinking water but also habitat for huge numbers of bats. As a result, Texas is home to 32 different species, the largest urban bat colony in the world (1.5 million under the Congress Ave bridge in downtown Austin), and the Bracken Cave colony, a nature preserve outside San Antonio with 15-20 million Mexican free-tailed bats. It's the largest gathering of mammals on the planet! Take that, Tokyo. 

The bats emerge at dusk above the permeable rocks into the fading sky. When they fly together from their roost, they form a chirping, immense black cloud, streamlined for a feast of mosquitos and insects. I often swim at that time in the spring and summer; I float on my back and watch them fluttering overhead, their jerky silhouettes in hot pursuit of pests and biters. I always wish the bats endless success in their evening hunt.