A Landing a Day

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Posts Tagged ‘meandering river’

The Brazos River, Texas

Posted by graywacke on June 4, 2018

First timer?  In this formerly once-a-day blog (and now pretty much a once-every-five-or-seven days blog), I use an app that provides a random latitude and longitude that puts me somewhere in the continental United States (the lower 48).  I call this “landing.”

I keep track of the watersheds I land in, as well as the town or towns I land near.  I do some internet research to hopefully find something of interest about my landing location. 

To find out more about A Landing A Day (like who “Dan” is) please see “About Landing” above.  To check out some relatively changes in how I do things, check out “About Landing (Revisited).”

Landing number 2404; A Landing A Day blog post number 838.

Dan:  Today’s lat/long (29o 57.515’N, 96o 6.455’W) puts me generally in southeast Texas:

My local landing map shows that I landed “adjacent” to the Brazos River:

Two things to say here.  You’ll notice the absence of towns in my title, and the absence of towns on my landing map.  Secondly, you may be curious as to why the word “adjacent” is in quotes above.  You’ll find out shortly.

The absence of towns isn’t because I landed in a desolate, unpopulated portion of Texas.  I was just zoomed in a little too far.  So here’s a more typical local landing map:

And yes, the towns are pretty much hookless.

I certainly don’t need a local streams-only map, but here’s a regional shot showing what happens to the Brazos River:

Oh yea, before I forget, this was my 33rd hit in the Brazos River watershed.

There was no decent Google Earth (GE) Street View coverage of my landing, but I could put the Orange Dude on a bridge over the Brazos downstream from my landing:

We’ll quickly get to the bottom of the “adjacent” issue (where my landing map showed my landing adjacent to the river) by taking a look at a very local Google Earth (GE) shot of my landing:

What’s going on here?  My StreetAtlas maps and Google Earth always are in precise agreement when it comes to the location of my landing.  Well, guess what?  From a lat/long perspective, they are in precise agreement!

You can probably guess what’s going on here.  The location of the river has shifted to the south!  I landed at the outside edge of a meander, and those outside edges have a way of migrating through time.  And this movement happens not on a geologic time scale, but on a human time scale.

GE has a nifty historic aerial photo tool, which allows us to take a close look at what’s going on. The shot above is dated February 2017.  Let’s go back in time a little, to November 2015:

Hmmm.  A little closer to the bank, eh?  How about April 2012? 

Not much change.  October 2008:

Right on the edge.  But check out April 2006:

No doubt about it!  Here’s September 2003:

And finally, February 1995:

I’d say the basis of my StreetAtlas map was the river’s location back in the early 2000s.  The Brazos marks a county boundary near my landing, and the StreetAtlas map shows the boundary line right in the middle of the river.  I wonder how they handle the migrating river issue?  Does the county boundary shift, or was it defined by a years-ago river course and now follows a meandering path that doesn’t always line up with the river?

So.  Why do rivers meander?  Most references have long, esoteric discussions that I generally find tedious.  Steep valleys underlain by bedrock don’t meander; their courses are generally straighter, and may be controlled by features in the bedrock.

But streams that are in unconsolidated sediments (sand and clay) which typically have a much flatter gradient, are much freer to meander.  I think that the simplest explanation is that “nature abhors a straight line,” and the slightest bend becomes more and more extreme.

Here’s a Wiki figure showing the typical progression of a meander:

The cut-off meander in the final stage is known as an “oxbow lake.”  Here are a couple of oxbow lakes just upstream of my landing:

The outside bend of a meander (where active erosion is occurring) is known as a “cut bank.”  The inside of the bend (where sand deposition typically occurs) is known as a “point bar.”  So, while one bank is being eroded, the opposite bank is building up with new sediment.  Here’s my landing location:

And here’s a shot showing how much longer my meandering river is than a hypothetical straight version:

It’s only a matter of time before a new oxbow lake is created just west of my landing.  What’s your guess?  Fifty years from now?

And the Brazos certainly does flood (when most of the cut bank erosion occurs).  Here’s the front page of an article from the Houston Chronicle:

That’s one ugly cut bank!

While I was working on this post, I received the following text message from my daughter Willow:

“Thanks for the good night.  [She and the grand kids were over for dinner earlier that evening.]  What’s the curvy river/stream west of Harrisburg that flows into the Susquehanna?”

I had no idea why she cared about the “curvy river/stream west of Harrisburg,” but figured that she had been on Google Maps (which doesn’t label streams), and couldn’t find the name of said stream.

Well, she certainly asked the right person.  I went right to StreetAtlas, and found said curvy stream.  I texted back:

“It’s the Conodoguinet Creek.  Wow.  It meanders like crazy!”

Here ‘tis, with all of the urban trappings:

Check out all of the residential neighborhoods sitting right within the meanders.  The stream can’t flood on a regular basis!  And it’s course certainly isn’t shifting like the Brazos.  My guess is that these meanders are “entrenched.”  More about that in a minute.  But here’s a streams-only shot:

And a distance-comparison shot (straight line, about 11 miles; meandering line, about 30 miles):

From Wiki:

Conodoguinet Creek is a 104-mile-long tributary of the Susquehanna River located in the greater Harrisburg metropolitan area.   The name is Native American, and means “A Long Way with Many Bends”.

So what about “entrenched” meanders?  These are meanders that came to be when a stream is meandering lazily along (likely not very high above sea level, like the Brazos), when regional uplifting comes along, and the entire system is raised.  The meanders hang in there, but the stream begins to cut down vertically, even vertically through bedrock.  This results in much higher topography in and around the meanders (allowing development like we see west of Harrisburg).

Here’s a GE shot showing the urbanization of some of the meanders:

FYI, the land between the meanders is typically between 70 and 120 feet higher than the elevation of the adjacent stream.

In the case of the Conodoguinet, the uplift occurred many 10s of millions years ago.  Check out this GE shot:

The Conodoguinet is in the right foreground.  To the left, see how the ridges have been cut by the Susquehanna?  This is all part of the same process.  Eons ago, the ridges weren’t there; the underlying linear sandstone formation had been eroded to near sea level.  The Susquehanna (and the Conodoguinet) were also near sea level.  When the whole area was uplifted, the Susquehanna cut through the underlying formations of sandstone (creating water gaps), and the Conodoguinet cut through the underlying shale formation – leaving the meanders in place.

Any questions?  If so, check out my Nanticoke, Pennsylvania post that covers water gap formation in much more detail (type “Nanticoke” in the search box).

Phew.  All of this Pennsylvania stuff because of Willow’s somewhat random text . . .

Anyway, back to the Brazos. From Wiki:

The Brazos River, named by early Spanish explorers Rio de los Brazos de Dios (translated as “The River of the Arms of God”), is the 11th-longest river in the US at 1,280 miles from its headwater source in New Mexico down to the Gulf of Mexico.

The watershed:

And I have landed twice in the NM portion of the watershed (including my Clovis NM landing which is worth a visit or re-visit).

So how does one pronounce “Brazos?”  East Coast elitist that I am, I simply assumed BRAY-zohs.  Wrong.  I encourage you to click HERE, scroll down until you see the audio file you can click on, and listen to a local who really knows how to pronounce Brazos!

I’ll close with this GE photo from a few miles north of my landing by Houston Suburban Warrior:

That’ll do it . . .

KS

Greg

 

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