Between the Black Hills of western South Dakota and the Pecos River in southern Texas, there's an expanse of open country that is still big enough to daunt a traveler, even if he's clocking 80 miles an hour with the air conditioning at full throttle. Every 50 miles or so, a river has cut a modest valley and slips off to the east, but between the drainages, the land looks as if it's been leveled by surveyors. In many places, it's so flat that a drop of water hitting the ground can't make up its mind which way to run -- it just sits there until it soaks into the ground or evaporates.

This is playa lake country. The playas are shallow basins covering as little as 800 square feet or as much as 1,200 acres. After a snowy winter or wet spring, they fill with water, much like the potholes of the northern prairies; without a steady source of local water, they shrink and the smaller ones dry up entirely. In a normal spring, there are at least 50,000 playas on the High Plains, almost 20,000 of them in the Texas Panhandle.

It's hard to say how many birds used the playas before the High Plains were settled. Few people visited this part of the world casually, and most of the ones who made the trip were too distracted by immense herds of bison to report on lesser game. In the summer of 1845, Lieutenant James Abert and his expedition visited the Staked Plains of the modern Texas Panhandle where they found "large shallow pools covered with flocks of ducks," which they did their best to thin. The playa lakes continued to attract waterfowl in the 20th century. Maps of winter range drawn in 1930 showed birds in the Texas and Oklahoma panhandles.

For generations, the main crop in the region was grass, but as early as the 1930s, farmers began using groundwater to irrigate crops. By the early 1950s, cheap energy, new technology and an ancient deposit of sand and gravel combined to transform the western plains. The deposit is known as the Ogallala aquifer. Stretching from Nebraska to central Texas and New Mexico, the Ogallala is a gigantic subterranean reservoir that once held almost as much water as Lake Michigan.

Some of this is fossil water. The rest is water that finds its way down from the surface through the sand of riverbeds or the bottoms of playa lakes. The recharge process is slow -- in a typical year about a third of an inch of water finds its way down from the surface -- but over the course of the millennia, the resulting reservoir of water is astonishing. In some places the saturated Ogallala gravel is hundreds of feet thick with water moving downhill through it like molasses at the rate of about 150 feet a year.

As it became practical to bring Ogallala water to the surface, a whole lot of ranches on the plains became farms. It's been estimated that 65 percent of all the irrigated acreage in the United States today is on the High Plains.

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