Figure by North Plains Groundwater Conservation District
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Playa Lakes are transient bodies of surface water that form in the arid, southern Great Plains. They are especially abundant in the “Playa Lakes Region” (PLR) of northwest Texas, western Oklahoma, eastern New Mexico, eastern Colorado and southwestern Kansas. There are 66,000 playa lakes in the PLR, and a third of these lakes are found in the high, tablelands of the Llano Estacado of West Texas and Eastern New Mexico.
The Canadian River, which cuts west to east across the northern tip of the Texas Panhandle and then runs all the way across Oklahoma, divides the PLR into northern and southern sections. Annual rainfall is slightly greater in the northern sections of the PRL and the climate is also slightly cooler. This causes the northern playa lakes to be larger and have longer periods of water retention. The smaller, more ephemeral lakes in the southern part of the PLR, though, are much more numerous and much more closely packed together .
There are some similarities between playa lakes and the “vernal pools” of eastern North America (see Signs of Spring 10, April 28, 2016 for a vernal pool discussion). Neither, for example, contain fish, and both are vital habitats for amphibian reproduction and unique communities of plants. Playa lakes, though, can stay dry for many years or even decades while vernal pools are almost always filled each year by seasonal snow melt and spring rains. Also, playa lakes can form any time of the year. They just require an intense rain event, something that occurs rarely but inevitably across these very dry climate zones.
Plants and animals that inhabit playa lakes, then, must have ecological and metabolic adaptations that enable them to tolerate extremely dry conditions while they successfully wait for the return of the lake waters. They must also be able to quickly respond and, especially, rapidly reproduce when the transient aquatic habitat returns.
Photo by USFWS, Public Domain
Playa lakes are recognizable even when they are not filled with water. They are saucer or plate-shape depressions in an otherwise astonishingly flat landscape. The periphery of a playa lake is a sloping annulus often rimmed by grasses on the upper margin, and the lake bottoms are clay-pans full of broad cracks when dry but solidly continuous when wet. Most playa lakes (87%) are less than 30 acres in surface area with potential water depths of about three feet.
A playa lake is the center of an independent watershed. It fills primarily via surface water flow from its surrounding drainage area. We discussed dry climate soils a couple of months ago in the Signs of Fall 6: the baked soils of an arid or semiarid ecosystem are coated with water-repelling hydrocarbons and have very slow water infiltration rates. Rainwater, then, flows rapidly across these surfaces into central depression areas and fills the playa lake.
It takes an intense burst of rain to fill a playa lake: two or three inches of rain in a few hours or, possibly, over a day or two. Heavy thunderstorms in the spring or summer are often the filling agents for these lakes. Depending on the ambient temperature, a significant volume of the lake’s water will evaporate (50% in the summer). Also, a great deal of the drainage water that first flows into the lake will drain down through the large cracks in the clay pan. Dry playa lake beds allow water to infiltrate into the underlying soil twenty time faster than do the surface soils outside the playa boundary. As the clays moisten, though, the pan seals up shutting off water infiltration allowing the lake to retain more and more of the inflowing water.
Fall forming playas persist longer than spring or summer forming playas. These fall playa lakes are vital stop-over points for migrating water and shore birds including Canada geese, sandhill cranes, plovers, rails, sandpipers and a variety of ducks.
clay pan in dry playa lake, USDA, Public Domain
Water percolating through the playa lake bed is extremely important because some 20% of this downward moving water will reach ground water. In much of the area of the PLR the ground water reservoir is the Oglala Aquifer. It is estimated that 80% of the annual Oglala recharge comes from playa lakes even though these lakes only make up 2% of the land area over the aquifer!. The clays in the lake beds also naturally filter out nitrates, sulfur, chlorides and even pesticide and herbicide residues thus greatly increasing the quality of the water entering the aquifer.
Sediments from the surrounding landscape can clog up gravitational water flow channels and interfere with the clay minerals in the lake bed thus reducing the ground water recharge percentages of a lake and also its ability to filter out impurities. The presence of grasses around the rim of the lake are quite important to filter out these surface transported sediments before they enter the body of the lake.
Why are there so many playa lakes in the southern Great Plains, and why are so many of these lakes on the Llano Estacado? The best answer that I could find is because of the incessant winds on this lightly vegetated, incredibly flat stretch of high ground. Anyone who has lived in or even visited this area knows that wind is a significant feature of this landscape! The winds, carve out and exaggerate even very slight depressions in the surface soils. Over time, the wind digs down through surface soil layers into the clay-rich subsoils beneath. These small depressions continue to erode especially in the extensive dry periods when the forming lake bed is exposed. Places with less wind or with more complex surface topography to break up the passage of the wind, would necessarily form fewer and less obvious depressions, and would, therefore, have fewer and less obvious playa-like lakes.
(Next week: the ecological community of the playa lakes, some memories of playas and observations on a “fossilized” playa in New Mexico!)