Bitter Lake National Wildlife Refuge has one of the most unusual—and breathtakingly gorgeous—geological features in the National Wildlife Refuge System: water–filled sinkholes in a desert–like landscape.
Monitoring the sinkholes’ behavior as they respond to current drought conditions could shed light on the hydrology, geology and biology of the world–renowned Roswell Artesian Aquifer. The aquifer is what makes 24,536–acre Bitter Lake Refuge a hyper–diverse oasis in southeastern New Mexico.
The sinkholes form after rainfall between the refuge and the Sacramento Mountains (60 miles west) seeps slowly into the underground aquifer. Over time, hydrostatic pressure and minerals in the aquifer’s hard water combine to percolate into the gypsum/limestone–laden rock above it, basically dissolving the earth’s surface from beneath until it collapses in.
The result is sinkholes punctuating the landscape. To make a climatically and geologically incorrect analogy, imagine that the earth’s surface is a frozen lake, the aquifer is the water below, and the sinkholes are naturally formed ice–fishing holes.
The sinkholes at Bitter Lake Refuge are 15 to 230 feet wide. They are 50 to 70 feet deep. The salinity, pH and temperature of their cold water vary. New ones form regularly; some are believed to be 5,000 years old. There are about 70 sinkholes at the refuge.
One of the most impressive is Inkpot Sinkhole in the 9,200–acre wilderness area. Refuge manager Floyd Truetken says that, standing next to Inkpot recently, he “couldn’t help but wonder, ?Did native tribes stand here as we are doing and look at this with wonder? Was there a religious or ceremonial aspect for them?’ I am just in awe of their formation. It’s not common to see large numbers of sinkholes with water in them out in the middle of a prairie”—a desert, really.
How sinkholes at Bitter Lake National Wildlife Refuge in New Mexico respond to current drought conditions could shed light on the hydrology, geology and biology of the world–renowned Roswell Artesian Aquifer. This sinkhole, the largest on the refuge) is called Lake St. Francis.
Credit: Bill O’Brian/USFWS |
Beauty aside, the sinkholes and hundreds of spring vents from the aquifer are the refuge’s life blood. They are one reason Bitter Lake Refuge and nearby Bottomless Lakes State Park together were recognized last year as International Wetlands of Importance under the Ramsar Convention. They are a reason the refuge is host to rare invertebrate and fish species, including the endangered Roswell springsnail, Koster’s springsnail, Noel’s amphipod, Pecos assiminea and Pecos gambusia.
All told, 28 fish species, 360 bird species, 57 mammal species, 50 amphibian and reptile species, and more than 100 varieties of dragonflies and damselflies have been documented on or near the refuge.
Last year, extreme drought shrank habitat for migratory birds on refuge marshes, stunted vegetation growth and appears to have, indirectly, affected the sinkholes because of increased agricultural irrigation demand. “The water level in those sinkholes is an indicator of the aquifer level. It rises and falls according to the pressure,” says Truetken. “During the summer irrigation season, the farmers are pumping water and the aquifer naturally drops, and there’s a corresponding drop in the sinkholes.”
Because some sinkholes have dropped to abnormally low water levels, the refuge is keeping an eye on them.
“There is a direct correlation between aquifer levels and sinkhole levels,” says U.S. Fish and Wildlife Service Southwest Region hydrologist Paul Tashjian. “What we don’t know is how low it can go and still supply viable habitat. We’re monitoring spring vents, spring runs and sinkhole levels and relating these to the aquifer levels.”
What hydrologists do know is that rainfall takes, on average, 30 to 50 years to travel through the rocks and exit from refuge sinkholes or spring vents. How quickly the aquifer recovers from this drought—as evidenced by sinkhole level—may tell hydrologists a lot about the aquifer’s characteristics.
What happens to micro–wildlife during and after drought recovery may tell biologists—including a Student Conservation Association intern inventorying Pecos assiminea snails—a lot about the unusual life the sinkholes and spring vents sustain at what Truetken calls “a hidden gem” of a refuge.
“You drive for many miles from all directions through vast grassland and you come to the Pecos River Valley—and here we are,” he says. “The sinkholes add a tremendous diversity to that. You just don’t see that in most parts of the country.”
