[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

[cdt-l] more on the Nature of the Divide, our tax money well spent

I received your e-mail inquiry in a roundabout fashion.  I am preparing=0D
a short piece on continental divides for eventual inclusion on the=0D
National Map web site.  One of my motivations for writing the piece is=0D
that I, like you, have been frustrated by the paucity of information=0D
regarding continental divides.  I am gathering information to address=0D
this oversight.=0D
I cannot provide you with a reference to your questions.  I have not=0D
found a suitable reference yet.  With respect to your questions:=0D
1)  Is the divide discontinuous?=0D
>From my perspective, no it is not.  It is more or less obvious in=0D
places, but not discontinuous.  Also, the "hydrologic" aspect of the=0D
continental divide includes subsurface movement of groundwater.=0D
Therefore, it is possible for water to fall on one topographic side of=0D
the continental divide, but to flow through groundwater processes to the=0D
opposite side.  I cannot say where this might occur; only that=0D
theoretically it is possible.  This phenomenon has led to the terms=0D
"topographic" and "hydrologic" divides.  The two types of divides are=0D
not always coincident.=0D
2)  Is there a continental divide where there are no mountains?=0D
Yes.  Mountains are not a prerequisite for continental divides.  In=0D
fact, one of the greatest myths about "THE" Continental Divide, is that=0D
it is the only continental divide in North America.  To the contrary,=0D
there are other continental divides, for example the one separating the=0D
Arctic from the Pacific Ocean and one separating the Arctic from the=0D
Atlantic Ocean.  Let's consider the latter for a moment.=0D
The Arctic-Atlantic (or Hudson Bay/Gulf of Mexico) continental divide=0D
(called the Northern Divide by some and the Laurentian Divide by many in=0D
Minnesota) runs through central North Dakota at an elevation around 2000=0D
feet above mean sea level.  It is very real.  Rainfall on one side of=0D
the divide will journey more than a thousand miles down the James River,=0D
Missouri River, and Mississippi River to the Gulf of Mexico.  Rainfall=0D
on the other side will journey a thousand mile down the Sheyenne River,=0D
Red River of the North, and Nelson River to Hudson's Bay.  There are no=0D
mountains in North Dakota.  But from the smallest rill in a wheat field,=0D
you can clearly walk the divide and see the headwaters spreading in=0D
opposite directions.  [Incidentally, the divergent routes from North=0D
Dakota to the Gulf of Mexico and Hudson's Bay are far greater than the=0D
distances from the Colorado Rockies to the Gulf of Mexico and the Sea of=0D
Because North America has at least three continental divides (some=0D
authorities suggest five, though two are of more contentious=0D
designation), it is probably more accurate to refer to the continental=0D
divide, which coincides with the Rocky Mountains, as the Great Divide.=0D
Technically, it cannot be "The" Continental Divide--a misnomer of=0D
3)  Is there a continental divide where there is no precipitation?=0D
Yes for two reasons.  The continental divide marks a boundary between=0D
watersheds.  On any given day, there may or may not be precipitation.=0D
It is not a matter of if it rains, but what happens to precipitation=0D
when it does rain.  Second, no place I know of on Earth has been=0D
permanently dry.  Indeed, much of the landscape of many "desert"=0D
environments was produced by fluvial (river) activity!  Climate changes,=0D
weather happens.  Arid environments support little vegetation.=0D
Consequently, when it does rain, the water runs quickly on the surface,=0D
completely unimpeded by vegetation.  The runoff is very effective under=0D
these conditions to mold and shape the landscape.=0D
I completed my graduate studies at Los Alamos National Lab and the=0D
University of New Mexico so I know well the area you mentioned.  I=0D
studied much of the Chihuahuan and Sonoran desert landscapes of New=0D
Mexico.  The precipitation might be sparse, but the landscape is replete=0D
with signs of fluvial activity.  It is short-sighted to define a=0D
continental divide on only present conditions, when it has controlled=0D
runoff patterns from past climatic events and will do so in future=0D
4)  Is the location of the continental divide arbitrary?=0D
Yes, I think it might be, but my opinion might be a matter of semantics.=0D
If you look at the continental divides, you will find that in several=0D
places the divides bifurcate (split in two).  Suddenly there are two=0D
continental divides running across the landscape though eventually the=0D
divide joins back together.  This is true in a couple places.  For=0D
example, the divide bifurcates in south-central Wyoming.  Drainage in=0D
this part of Wyoming is internal, that is, it does not flow out to any=0D
ocean.  In North Dakota, the Devils Lake basin is internally drained and=0D
does not flow to any ocean.  Same with much of the Prairie Pothole=0D
region of North Dakota (locally called the Missouri Coteau--see the web=0D
page for the North Dakota Geological Survey).  But climate can change=0D
and these internally drained basins could theoretically fill up and then=0D
spill over into a new basin.=0D
Also, tectonic movements can change the position of the divides, so they=0D
should never be thought of as a permanent, static feature.=0D
In North Dakota and Minnesota, the Laurentian, or Northern, Divide,=0D
shifted repeatedly as glacial ice sheets blocked some drainages and=0D
rerouted others during the Ice Age.  For example, glacial Lake Agassiz=0D
(former lake that covered the border region of Minnesota and North=0D
Dakota) today drains north through the Red River of the North to=0D
Hudson's Bay.  When glaciers blocked its northern outlet, glacial Lake=0D
Agassiz filled to a higher elevation and spilled out to the east.  Lake=0D
water drained through the Great Lakes and St. Lawrence to the Atlantic=0D
Ocean.  Another glacial advance blocked the eastern drainage route too,=0D
so glacial Lake Agassiz filled even higher until it spilled over and=0D
drained to the south through the Minnesota River valley to the=0D
Mississippi River and to the Gulf of Mexico.  One lake, draining to=0D
three different sides of the continent (north, east, west) during a span=0D
of a few thousand years!=0D
Also, the weight of the ice sheets--some more than a mile thick, caused=0D
the earth's crust to sag.  Sagging in some places, combined with crustal=0D
rebound when the glaciers melted, has caused the position of continental=0D
divides to change continuously, though perhaps imperceptibly in human=0D
Is the position of the divide arbitrary?  Along some parts, I can=0D
imagine geographers pointing to broad zones a few feet to a few tens of=0D
feet wide, where they might say, "The divide is here, but it's=0D
impossible to put a finger on it."  In some places, the divide might be=0D
tens of feet wide.  In others it may be a knife edge, like the col=0D
between Grays and Torreys Peaks in Colorado, or the ar=EAtes along the=0D
Cirque of the Towers in the Wind River Range.=0D
I thank you, Caleb, for raising some intriguing questions concerning=0D
continental divides.  I'm not sure if I answered your questions to your=0D
satisfaction, but your questions gave me an opportunity to reflect on=0D
some of the things I have recently learned about continental divides.=0D
As I am currently writing on the subject, please do not hesitate to=0D
contact me if you have additional questions.  I cannot promise to answer=0D
them, but your questions may well inspire me to research a new topic for=0D
my paper.=0D
Mark A. Gonzalez, Ph.D.=0D
North Dakota Geological Survey=0D
600 E. Boulevard=0D
Bismarck, North Dakota  58505=0D
E-mail:  mgonzale@state.nd.us=0D
Phone:  701-328-8000=0D
Fax:  701-328-8010=0D
-----Original Message-----=0D
From: Brigitta Urban-Mathieux [mailto:burbanma@usgs.gov]=0D
Sent: Tuesday, December 17, 2002 11:58 AM=0D
To: mgonzale@state.nd.us=0D
Subject: Request for information about the US Continental divide=0D
It was good to speak with you earlier.  I look forward to seeing your=0D
article later in 2003.  Thank you very much for being willing to contact=0D
Caleb Ewing.=0D
Please feel free to contact me at any time.=0D
Gita Urban-Mathieux=0D
US Geological Survey=0D
Phone: 703-648-5175=0D
Fax: 703-648-4165=0D
Email: burbanma@usgs.gov=0D
National Atlas: http://nationalatlas.gov=0D
-----Forwarded by Steven Kambly/NMD/USGS/DOI on 12/17/2002 09:46AM -----=0D
To: <skambly@usgs.gov>=0D
From: "Cal Ewing" <calebe@fdn.com>=0D
Date: 12/16/2002 09:02PM=0D
I recently sent a query to several USGS geographers and hydrologists for in=
fo about the Continental Divide, below is the best and most recent reply. T=
his is especially for you Rafi, who dared to ask.=0D
Subject: Request for information about the US Continental divide=0D
Dear Mr. Kambly,=0D
I am looking for an overview or an analysis of the hydrologic data=0D
pertaining to the US Continental Divide. As you are listed as one of the=0D
originators of the USGS Continental Divide Map, I am hoping that perhaps=0D
could point me in the general direction where I might find such=0D
My interest in the Divide stems from my time backpacking along the=0D
Continental Divide Trail. I understand that the Divide is a physical=0D
reality in most places, but it is also discontinuous and perhaps even=0D
arbitrarily designated in other places as well, for instance, in the=0D
area of New Mexico where there are neither mountains nor water. The=0D
here and in other places seems to be as much a concept as it is a=0D
If you could recommend any scholarly writing on this topic I would be=0D
Sincerely, Caleb Ewing=0D