The Chihuahuan Desertâ€™s northern boundary is the Pecos River. In places the Pecos is laden with salt. How does the river become naturally polluted with these salts?
From its source on the east flank of South Truchas Peak, New Mexicoâ€™s highest peak, the Pecos River descends noisily through the domain of mountain species such as the marmot, a large groundhog and the dipper, a sparrow sized bird that walks under water. It rushes past the old Pecos Pueblo from which it acquired its name. In the past, itâ€™s had different names like Rio Salado, salty river and Rio Puerco, pig or dirty river.
From the mountains, the river comes out on a flat land of red shale and sandstone laid down by ancient streams in the early days of the dinosaur. There it picks up red mud from overgrazed pastures and a few plowed fields – natural in a country where torrential rains can follow droughts. The water is still good enough to support a stand of cottonwoods as far along it as a few miles past Fort Sumner, New Mexico.
North of Roswell, New Mexico, the Pecos begins to pollute itself by dissolving salt and gypsum and Permian rocks. Lining the river there are alkaline lakes and sinkholes now called the Bottomless Lakes. For hundreds of miles southward there were originally only a few mesquite thickets bordering the river.
Early explorers described a swift muddy salty river 65 to 100 feet wide and 7 to 10 feet deep. Near the present town of Grandfalls, Texas were falls 5 to 8 feet high. One of the few fords was Horsehead Crossing in Crane County, Texas, where horses died after drinking the water. The Indians, too, avoided the saline river.
But the lower river is much cleaner, as the chemistry of the limestone rocks of the canyons removes much of the salt.
In Texas the first water was taken for irrigation in Pecos County in 1870. The first canal system was constructed at Barstow in 1889, and, by 1905, the area was famous for its orchards and vineyards.
But by 1911 the fruit crops began to decline due to excessive salinity, and by 1918 the ruin was complete. Since then cotton and alfalfa, which are resistant to alkali, have been the main crops.
The soil along the river has for ages contained a large amount of alkali due to flooding by the salty river. At times when there is less water from the mountains the middle Pecos, the area from north of Roswell to Iraâ€™an, Texas, the water is exceedingly salty.
Around 1914, someone introduced the salt cedar (Tamarix), a native of dry alkaline spots in Southeastern Europe. The tree has spread wildly. It now lines the banks of the Pecos, and its long roots use up much river water. Sadly, it has displaced native salt-resistant vegetation. When the deciduous salt cedar drops its needles, the salt laden leaves make the soil even saltier. Recent millions-of-dollars eradication attempts have proven fairly ineffective.
Most of the tributaries from the west donâ€™t contribute to the Pecos, except in times of flash floods. The huge Comanche Springs at Fort Stockton used to flow 8 or 10 miles toward the river, 50 miles away, then vanish downward in a cattail swamp two miles long and a quarter mile wide. The earliest historical records donâ€™t mention water beyond that point. The spring water was diverted for irrigation and wells drilled in the area killed it. The cattail swamp dried up, caught fire, and smoldered for several years in the 1940â€™s.
When you see the Pecos River, stop, look out over the salt cedar and pickleweed along the river and the creosote bush beyond. We live in an arid land where even this water is precious.