Photo Essay
Waterholes of the Dunes
Malcolm McElvaney has photographed the waterholes of the sand dunes near Monahans for two years. He finds the waterholes by using Google Earth. He finds dark spots among the dunes and records the GPS coordinates, and then sets his GPS to find them. He has found over 20 waterholes, visiting them in all seasons of the year. One of his photographs proved to establish a new range extension record for a wildflower, catching the interest of professional botanists in the state.
Sand dunes capture rainwater because of the many air spaces between the sand grains. The water penetrates about four feet down, (but no further because of osmotic pressure) then begins to move down slope. Green algae, euglena and other microinvertebrates are able to survive in this wet zone under a sand dune. The water continues downslope. Underneath the dune field is a hardpan soil that slows the further penetration of the water down into the ground. In between the dunes, dry hardpan sometimes can be found, but in other locations waterholes form. The waterholes, with their immense annual organic content production that slowly decays into clay, then forms clay "lenses" which slows the penetration of water even more.
Some of the waterholes eventually are covered over by moving sand dunes and disappear, but others seem to follow a different scenario. After viewing Malcolm's photographs, the staff of the Sibley Nature Center has developed a theory that the waterholes have a "life cycle." As the area develops the clay lens and vegetation matures, so do the waterholes, until in some waterholes the plants utilize all of the water and open water can no longer be found except after major rain events. As of yet, the Sibley staff has not found any published research that might prove or disprove the theory.
Some of the permanent waterholes may be along draws (watercourses) long buried by the sand except in isolated spots. As the sand dune field is a vast "catchment" area, the excess water in the dunes may slowly drain out of the dune field to fill the permanent waterholes. It may take decades for this "excess" water to move down the hidden watercourses.
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Some of the waterholes are "animal wells." When a waterhole dries up during a drought, coyotes and other animals will begin to dig down to reach the water. Sometimes the animal wells will be four or five feet deep, angling into the ground at a 45-degree angle.
Permanent waterholes have cattails growing in the water. These waterholes never dry up, even in the worst of droughts.
Seasonal waterholes have bulrush and sedge growing in the clay. During dry times (and usually during the summer) no open water can be found. These waterholes will have water almost every year, after every substantial rain, and the water will remain visible for several months in the proper conditions.
Malcolm keeps records of the date of each photograph. Over time, he or other researchers utilizing his material may be able to determine the life cycle of the organisms (and the timing of the life cycles throughout the year) that utilize the waterholes. For the purposes of this photoessay, we merely report on what he has found, and do not often indicate the season. This water spider was at a permanent waterhole, but he has found it at seasonal waterholes, too.
Water striders of several species are found at all of the waterholes.
One of the water spiders skated across the water surface. The green algae on the bottom of the pond is one of many varieties of aquatic algae found in the waterholes. This species sometimes will be found in conjunction with bluegreen algae that form mats that sometimes rise to the surface, made bouyant by the gasses of decay.
Backswimmers were found in waterholes full of decaying organic material in the fall.
Small red bloodworms continually emerged and disappeared into a thick layer of organic material underwater. Some of the holes in the substrate are of their making, but some of the holes might be where the gasses of decay may be escaping from the substrate.
In the wet sand next to a waterhole, a small rove beetle scuttled past "Lemna," an aquatic plant that usually floats in water.
The water of the dunes brings wildlife of all varieties. Honeybees (probably Africanized) swarmed around a waterhole on a hot day. They landed on filamentous green algae exposed by the gradual drying of the waterhole.
Most people do not associate grasshoppers with water. In the waterholes of the sanddunes, a parasite of grasshoppers is common at times. The parasite is known as "hairworms." Hairworms are aquatic, but their larvae live inside of a grasshopper's "vitals." As the hairworm matures, does it stimulate the grasshopper to visit water? (Or is the return of the grasshopper to water merely random?)
Sibley staff members are unsure of the identification of these small flies that gathered in large numbers on the surface of the water. They might be a species of shore fly.
Cattails sometimes are shredded by hordes of these caterpillars. The adult moths are white, with a hairy thorax that makes them appear humpbacked.
Small spiders formed webs shaped like small bowls in the bulrushes of a seasonal waterhole in the fall.
Like all photographers, Malcolm took a turn at depicting the architectural aesthetics of a dew covered spider web!
A Twelve Spotted Skimmer, Libellula pulchella, perched over one of the waterholes. As far as Sibley staff can determine, no checklists of Odonata exist for the sanddune habitat west of the Llano Estacado.
Why did a deer die at a waterhole? Had it been injured or suffering a disease, or eaten something poisonous? Sedge and bulrush are not poisonous to grazers or browsers, although the silica in the plants can be irritating to their digestive systems.
Marsh Wrens visit the waterholes of the sanddunes every winter, but prefer the permanent waterholes with cattails that provide plenty of cover.
These just germinated cattail seedlings are red. Was the coloration a result of the water being cool?
In fall, black willow leaves begin to turn before the cattail seedheads begun to "fluff out" and disperse their seeds.
A tiny damselfly zoomed over one of the permanent waterholes with a dense stand of Chara algae. Chara smells like garlic when it is removed from water. It can form dense mats over a foot thick in permanent water.
Filamentous algae usually grows in shallow water.
The zonation of a waterhole is easily seen. The shore has sedges. In the shallowest waters, animals coming to water often keep a zone of bare mud, then filamentous algae rims a slightly deeper part of the pond, and chara algae grows in deeper water. The rhizomatous spread of the sedges can be determined by the lines of growth.
Toad tadpoles are found in the seasonal waterholes.
One of the tadpoles (on the left) has developed legs and moved to the shore to disperse. It will feast on termites and then disappear underground until the following year.
Standing water can be a deathtrap for insects. This predatory green lacewing eventually drowned.
Another species of water strider skated along the surface of the water.
A bird caught a dragonfly and snipped off the juicy abdomen, leaving the inedible wings to float in the water and become part of the organic load of the waterhole.
Water boatmen also seem to prefer waterholes with plentiful organic material.
Take a close look - can you see the shorefly and the horsehair worm among the bubbles of escaping gases, and the oily scum from decaying organic material?
Some of the smallest waterholes were full of organic scum.
Some of the scum was quite thick. Why does some of the scum float, and some not?
Under an overhanging sedge stem, the water surface was littered with the carcasses left behind by a dragonfly that returned to the stem after every successful hunting foray.
In the fall, daphnia swarm in large numbers. The fall generation will leave behind eggs for the following year.
Why was a rodent skull underwater?
Why was a beetle resting on a log in the middle of the water, a log with a sheen of green algae?
In the fall, black willows glow golden in the morning sun at the waterholes of the dunes.
Bluebells (Eustoma grandiflora) are often found near water across much of the state.
At one of the waterholes, Malcolm found a species of wild bean (Phaseolus). Sibley staff has not determined which species, yet.
Marsh fleabane blooms in August each year, and is another common wet soil plant in West Texas.
Buchnera americana had never been seen in the sand dunes before. Only one specimen was found. Check out the Waterholes of the Sand Dunes essay to learn more.
Cocklebur is common in the sand dunes, and is often found far from any water, which indicates how moist the dunes can be. Normally cocklebur grows in low places where water stands after rains (around playas and in draws.)
Russian olive is not native to the sanddunes. A bird ate a fruit elsewhere and deposited the seed when getting a drink. It is sometimes used as an ornamental tree in home landscapes. In northern New Mexico and further north it has become a riparian pest, much like salt cedar has in West Texas.
In the foreground is seepwillow, a shrubby daisy. To the left is rabbit's foot grass, another European ornamental that commonly is found in riparian locations. Black willows are seen beyond the bulrush filled bottom of the waterhole.
Seepwillow seeds gave a perch to a leaf footed bug with its proboscis tapping the resiny phyllary. The moisture found might be for sustenance, or it might be part of the chemical defenses of the bug.
A large preying mantid used the shady side of a black willow trunk for a hunting perch during a hot day.
Paper wasps strip black willow bark to make their paper nests.
In a clump of grass near a waterhole, a porcupine pretended to be invisible.
In one of the large cottonwoods found near a few of the waterholes, winter defoliation revealed two swinging oriole nests.
Black willows ring a bulrush filled waterhole.
Feral hogs keep this permanent waterhole torn up by their wallowing. The large leafed plant is a cocklebur. Shrubby seepwillows and mature willows provide cover.
In the bottom of one of the waterholes two generations of seepwillow can be seen. The older generation, now dead, are in the bottom of the depression, while the younger generation lines the edge. What killed the older generation? Did a rainy year keep the ground too saturated with water for its survival? Did a fungal growth kill the older generation? Did the soil composition change and the seepwillows optimal conditions disappear over time?
Some of the waterholes have active dunes slowly encroaching upon them. The willows will survive, continually putting out new branches to stick above the sand. A willow with five feet of above ground branching may actually have a dozen feet of trunk swallowed by the dune. Coyote willow, a different species of willow, will send out rhizomatous runners so that many "suckers" will pop up. Do the sanddune black willows have some coyote willow genetics and do the same?
One of the permanent waterholes had the floating algae association mentioned above. Livestock and feral hogs may limit the growth of cattails, and add their waste to the water, which then gives the ideal conditions for the floating algae.
Willows and russian olives line a permanent pond with cattails, next to a mostly vegetated dune.
Is this an old waterhole, near the end of the period of permanent water? The willows are large, and grass now grows under the willows. The bulrushes are of an immense size - six and seven feet tall, instead of the usual three to four feet.
A moving dune slowly encroaches upon a waterhole. The evidence are the half-buried bulrushes on the far shore.
This might be another "old waterhole." The willows are far from the water, and grasses fill the area between the willows and the bulrushes, which surround a small amount of open water.
Are the willows "climbing the dune" by rhizomatous growth, or are they buried older trees?
What role do feral hogs play in creating the landscape of the older waterholes. Settlers released hogs to forage in the shinoak covered dunes over a hundred years ago, but only in the last 15 years have their numbers begun to skyrocket. Why are feral hog populations exploding now, and not in the past?
Is this a very old waterhole? Did the willows begin to take all of the water, allowing the grasses to fill the bottom of the depression. Note that the large bluestem grasses (normally found on the sanddunes) have begun to grow in the bottom of the depression, too.
Waterholes are easy to spot from a distance, for the willows stick up above the surrounding dunes.
Is this a "young" waterhole, with small willows, some sedges, but plenty of open sand dunes around it? Or is it a seasonal waterhole that will remain looking as it does now for a long period of time?
No water was found under this huge old cottonwood.
In the summer, porcupines love the old cottonwood. Their major predator, mountain lions, are found in the dunes. The mountain lions normally eat the deer of the dunes, but know the trick of sticking a paw under a porcupine to flip it over to kill it, when the deer are hard to find.
In winter, the old cottonwood has no aquatic vegetation underneath one side of it. Another side has a small area of aquatic vegetation. After a significant rain, water did pool up in the area with no aquatic vegetation, but, none grew there, and the water disappeared in less than a month.
Notice the ring of bare sand between the ring of willows and the ring of large bluestem grass inside of the ring of bare sand. Bulrush is only found near the small pool of open water. Is this a "middle-aged" waterhole"?
The fact that no cattails grow in this waterhole indicates it is a seasonal waterhole.
During the winter Malcolm turned over one of the old willow logs at a waterhole and found a peculiar fungus with a weblike structure.
Near the webby fungus, waterstriders were overwintering under the log. Were they feeding on the fungus, or tiny insects that live on the fungus. To find overwintering water striders was a surprise.
A lesser goldfinch sported spring plumage in February as it perched in a leafless willow.
In the winter dragonflies can sometimes be found near the waterholes.
Wolf spiders with egg sacs were plentiful in the winter, too.
Jerusalem crickets live underground, emerging to forage for food at night. What stuck this one to an old sunflower stalk? Jerusalem crickets have been seen in early December, but the photo was taken in January. Had it been there for over a month?