The Pecos and Black Rivers


What does knowing something about the Pecos and Black Rivers in southeastern New Mexico have to do with your quality of life?

Nothing is the most likely answer.

So why should you care?

You do not have to care.  

BUT, let's pretend you are a tourist driving south from Carlsbad along the National Parks Highway.  You are, no doubt, going to see  one of the world's true treasures and enjoy the underground splendors of  Carlsbad Caverns.  

If you are a real outdoor enthusiast, you may also continue south and take in parts of Guadalupe Mountains National Park.

As you drive south on this highway, to the east of both Carlsbad Caverns National Park and the Guadalupe Range, you will pass over something called the Black River.  At 65 miles an hour (I am presuming you do not speed) you will likely miss it altogether.  

Here is a map that shows where these rivers are with respect to the National Parks Highway (US Highway 62/180):

But as you look right, occasionally and carefully, you may wonder what lies in the desert between you and that growing mountain range, which rises to your right as you go south because it has its highest peaks at its southern end, in Texas.

If you peruse these linked pages, before you drive south, NOT while you are driving, you will be able to tell your passengers the story of the little, short, Black River, its gypsum origin and loading, how it appears and disappears along its length, and where it finally ends up: adding its flow to the Pecos River.

Unless your passengers are sound asleep at this point, or maybe especially if they are -if that was your goal, you can continue with the story of the very long Pecos River just a few tens of miles to your left, and how different it is in terms of origin and how it gets sabotaged just to your left by upwelling brines from a karst area to its north, Nash Draw, that makes it quite a salty stream as it flows into Texas.

Perhaps they will wake up craving salt (chips anyone?).  

Or not craving salt (sodas anyone?).

Of course if they wake up asking for food or drink you will also seize this new moment and tell them the story of the limestone caverns carved out by underground waters, and you might just mention how caves relate to karst, and explain karst, the new word for the day.

And as you head further south you will no doubt regale any renewed snores with how at one point this was land's end and these mountains were underwater-reefs  at the edge of lobes or bays of the single sea that surrounded the world-continent Pangea, 250-million years ago at the end of the Permian, before the dinosaurs!

In other words, if you peruse these pages on southeastern New Mexico, you will learn "stuff" you can pass on to your captive car audience.  

Only if you have a rather mature audience, you can tell them about the ancient Madrone trees of the Guadalupe Mountains.  Trees from an ice age ago that strip for you, and provoke thoughts of smooth, naked surfaces decorated by color-rich leaves and berries, trees that dance seductively for you in the slightest hint of a breeze.  

If you are on this trip trying to woo a lover, this may help set the mood.  Or if your derring-do in these mountains causes you to get dumped by your would-be lover, maybe a dance with a naked tree can make you feel loved and lovable again.  

Naked-tree-therapy (my copyright as of this moment, please send me money if it works for you).

That is why you may want to peruse these pages.

(This is a limited-liability website, if your traveling companions are not intrigued by your local knowings, or your would-be lover is turned off by your geo-biological facts, don't blame me.  Please.)

The Pecos and Black Rivers: Background Discussion

Nash Draw, the depressed karst terrain just northeast of the Pecos River near Malaga, New Mexico, was fascinating to me. Its half-million year history is interesting, as is its rather surprisingly sound and resistant eastern boundary where competent redbeds and caliche keep water from entering the deeper formation that is composed of a series of salt layers deposited by the evaporating waters of an ancient sea that spilled water into a semi-confined lagoon on its boundary where it evaporated.   The mountains of southeastern New Mexico and adjoining parts of west Texas, many of them, are remnants of a reef system over which this sea water spilled steadily or intermittently, when sea levels started to drop locally because land was slowly lifted up.

Nash Draw formed as water entered its deep salt layers through somewhat less competent overlying sediment and rock layers. Here these layers only lasted about 224.5 million years, whereas to the east and north and west they have lasted 225 million years and are still competently protecting the salt underneath.

Maybe eventually they will also give way? Not in human time, but perhaps in distant geologic time. Even with some sinkholes and rainfall-caused erosion along the boundary of the draw, it takes serious geologic time to make significant changes to the size and shape of this feature. Estimates are that it has had pretty much this current shape and size for well over 400,000 years (for simplicity I round that up to a half million years). So, whatever degradation has been happening along the boundaries, it is very, very slow.

Within the draw, as salt dissolved several hundred feet below, the ground above sank down just over a hundred feet, generally. The draw was not formed by the usual wearing away of surface layers by flowing waters. Hence it does not look like a canyon. It isn't a canyon.

Nash Draw is a small "closed" basin, meaning there is no surface outflow. So why does it not fill up with water and overflow to the south, its lowest area?


First, there is not enough water coming in, given the dryness in Winter and heat in Summer. Water evaporates from the catchments in the basin as fast as it comes in.

But that is never the whole story, because when inundated by an unusually wet storm, there is water added to the system and both above-ground and underground water-levels rise. Some of that will evaporate later, leaving salt behind, but some of that also flows down, and then out toward the south where the Pecos River tries to sneak past unnoticed.

A closed basin is said to be one that is "internally drained." That means surplus water goes down, and then out, below the surface. It doesn't mean the basin is water tight (there are exceptions, like Death Valley, a classic closed basin, where the downward flow stops at a certain depth and all incoming water is forced to stay on the surface and is evaporated, except during ice ages when there is a very large lake in the valley. Hence the phenomenon of its salt flats, like at Badwater, becoming lakes part of every wet Winter. To see this lake in Death Valley, with my granddaughter Aubree walking in it, please go here.

But Nash Draw is not as hot, nor as dry, as Death Valley, and its downward moving water follows an aquifer, a water-bearing zone often called the "brine aquifer" because of the salty nature of much of its water. The brine aquifer flows (underground) to the south, not to the southwest where the closest approach of the Pecos River lies.  

The brine aquifer is capped by competent overlying rock and sediment south of Nash Draw.  This competent rock cap does not allow the water to move upward from below, so the water is under increasing pressure as it continues to move gradually downward and south.

When that pressurized, briney water comes under the Pecos River, which has eroded away some of the surface rock and sediment, its pressure is sufficient to push some of its briney water directly into the river from below, and indirectly through flows from artesian wells at the river's margin.

The Pecos River deteriorates significantly, in terms of its water quality, as it receives this brine inflow from beneath and from its sides as it goes around Malaga Bend.  There are two bends east of Malaga, actually, as you can see in the next map, courtesy of MapQuest with a few added labels:

(What are all those white dots?  Oil wells! And there are still more being drilled.)

In the 1950s there was a study that suggested the river could be made more useful downstream, for crop irrigation, if a large well or several smaller wells could be placed into this pressurized brine aquifer and suck up enough brine to simply lower the pressure in that aquifer. Without the pressure, it would not shove its brine into the river nearly as effectively.

Great idea. It was tried, on an experimental basis, in the 1970s. For several years brine was pumped out of that aquifer, significantly lowering its pressure. The river was noticably improved in terms of water quality downstream. There was some worry that lowering the pressure too much could cause river-water to sink into that aquifer and lower the river's flow, but that was not observed.

The brine was pumped from under Malaga Bend and transported into a basin (Queen Basin on the map) about a mile and a half away, where it was allowed to evaporate. The idea was to let this run for perhaps a century and then find a new area to dump brine while the salt is harvested from this area.

Brine was also taken to oilfields in Lea County, New Mexico, and to oilfields just over the border in Texas. Brine is injected into oil fields to push oil in a desired direction where a well is waiting to pump the oil out of the ground.

That brine removal worked fine, but then the pump and well casing gave way, and the experiment was over. In 2000 and also even more recently, the New Mexico and Texas River Commissions both asked the federal government to do this again, this time for the long term and not just as an experiement.

There was also a problem, once the system shut down, selling the salt. But when I visited the Queen Basin where just a few years' worth of salt had accumulated, there was work in progress to remove the salt: it had been sold.

One thing I like about New Mexico is that when a word like "Queen" or another royalty term is used in a place name, it does not mean the English royalty for which many Eastern US places are named. In New Mexico these place names refer back to Spanish royalty.

Interesting (to me, but I am obviously easily amused).

Just before the Pecos River runs into this brine trap, it is joined by another desert river, the Black River. This one is very different from the Pecos. The Pecos is a river that starts in high snowy mountains far away in the north of New Mexico, and flows way over 400 miles southward until it meets the Rio Grande, which starts in the next valley over in the same mountains, north of Santa Fe.

The Black is different, because it starts with some seeps and springs in gypsum-rich hills east of the Guadalupe Range. Its main line is not fed through runoff from the Guadalupe Range, as one might expect, except during infrequent significant rain events over the Guadalupes.

During such downpour events there is flow in the canyons leading out of the range, and some of that flow has carved sizable canyons all the way to the Black River, and probably contributes most of the flow in the river during and just after those events.

But we are visiting in Winter, the dry season here, so all we will see is "base flow" --seeps and springs make up base flow, there is no water coming in from overland-flow caused by rainfal or snowmelt during this time.  Winter is the driest part of the year in the Chihuahuan Desert.  

The map above has it right, the Black starts below and beside the Guadalupe Range, not on it. (It starts where the number 16 is).

The Black River is only about 40 miles long, a very short river compared to the lenghty Pecos. It starts out, and finishes, either saturated with, or only slightly undersaturated with, gypsum, the stuff they use to make wallboard for homes.

When water is saturated with a salt like gypsum, and it evaporates as it flows in the sun, it will deposit gypsum all along its flow-path. As springs downstream from the river's starting-point add water to the flow, the water quality becomes such that in some areas vegetation is abundant on the river bottom.

In one place this has a rather dramatic effect: a river bottom has a hole with water coming up yhtouh it from below.  It is covered with gypsum in the upstream direction, and sports some green growth downstream as the water flow increases from this added spring inflow.

Obviously I am easily amused, easily fascinated, and easily entertained by Mother Nature. Most real people I know drive very fast along here to get to where they are going, which is not here.

What we will do now on the next bunch of pages is take several walks along the Pecos and the Black rivers.

We do this for entertainment. And for exercise.

To the Pecos River we go!

Or. to the Black River we go!

The flow of the story was written in the Pecos=>Black order, but it really doesn't matter which river you visit first: there will not be a quiz at the end of your reading.

If you do not want to go to either river just now, please use one of these three links to return to the page that got you here:

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