Southwest Railroad History

TECTONICS AND EROSION

The story of railroad development in the arid Southwest is the story of transcontinental railroads constructed across a barren and beautiful land to connect the east and west coasts of the United States.  These east-west routes had to contend with north-south-trending topographic barriers, most notably the Continental Divide and the Pacific Crest.  These divides are barriers not only to railroad construction but also to humid air, which is rung dry of its moisture when it rises over the Continental Divide and the Pacific Crest.  The dry land in between -- the Southwest Desert -- held its own barriers to railroad construction.  Most of this region is in the Great Basin, which encompasses almost all of Nevada and adjacent areas.  This region of internal drainage is characterized by north-south-trending mountain ranges, hundreds of them, with elevations up to 14,000 ft, and intervening north-south-trending valleys, with some elevations below sea level and many with dry or saline terminal lakes, most notably Great Salt Lake.  The relatively flat Colorado Plateau region of northern Arizona, eastern Utah, and adjoining parts of New Mexico and Colorado, wasn’t much easier for railroad construction; the problem here wasn’t mountains, but rather canyons, such as the Grand Canyon and its tributaries, and cliff-bounded plateaus with elevations up to 13,000 ft.

 

While earth tectonics throw up obstacles to railroad construction, earth erosion by flowing water creates corridors through the tectonic obstacle course.  All forms of transportation -- foot travel, wagons, canals, railroads, automobiles -- follow watercourses, which provide a constant grade (without ups and downs) from the source of the watercourse on a local or continental drainage divide to an ocean (or inland sea or terminal lake).  Additionally, watercourses have flat floodplains for easy travel and are the locations of human settlement due to the availability of water and the rich soils on floodplains.  

 

DRAINAGE DIVIDES

The close relationship between railroad routes (and other types of land travel routes) and land drainage can be visualized by imagining being tasked with building a transcontinental railroad.  Every continent has a continental divide, which is a continuous ridge separating the drainage (slope) to one ocean from the drainage to another ocean. Every continent has at least one continental divide, and the only complications to this rule are (1) defining what is an “ocean,” given there is really just one “world ocean,” and (2) the case of internal drainage basins, which do not drain to the ocean.  The only continental divide in the Southwest is the "Great Divide" separating the Pacific drainage (also known as a "watershed") from Atlantic (including Gulf of Mexico) drainages.  Internal drainage is important in the Southwest, including the Great Basin and two basins along the Great Divide. 

 

By definition, one side of a continental divide drains to one ocean and the other side drains to another ocean.  So, if you stand on the divide and start walking down either side, following the first stream and continuing down to the stream it feeds, eventually you will reach the ocean and have travelled downhill the entire time.  In other words, if you follow the stream and never climb the slope of the stream valley, you can survey a route that climbs continuously from one ocean to the continental divide and then down continuously to the opposite ocean.  The entire transcontinental route will have just one summit which, all else being equal, is the most efficient railroad route.  For a given crossing location (pass) on the continental divide, there is one optimal transcontinental route – the route prescribed by the drainage routes in both directions from the divide – and any other route means climbing out of a stream valley and creating another summit on the route.

 

The principle of following the stream applies not only to transcontinental lines but also to shorter routes.  For any railroad route between point A and point B, there are two possible scenarios.  The first scenario is that A and B are along the same stream and a railroad can follow the stream with no route summits – either A or B will be the high point on the line.  Such is the case of the Denver & Rio Grande Silverton Branch in southwestern Colorado, which follows the Animas River from Durango (low point) to Silverton (high point) with no summit.  If A and B are not on the same stream, there must be one or more summits on the route, and each summit and each intervening low point means decreasing efficiency, possibly including the need to add and remove helper locomotives.

 

If this approach to railroad construction were strictly applied, the optimal transcontinental line in the Southwest would be one that does not exist!  The Pacific end of this hypothetical line would be at the mouth of the Colorado River on the Sea of Cortez. The line would follow the Colorado River north to Yuma, then follow the Gila River eastward roughly parallel the Southern Pacific Sunset Route over the Great Divide in southwestern New Mexico to El Paso, then follow drainage down to the Rio Grande through El Paso to the Gulf of Mexico at Brownsville. This route would have one summit, but was never built because other factors go into railroad (and other travel modes) route selection.  One factor that controls route selection is the locations of the people and resources that railroads are built to connect.  For example, although none of the transcontinental surveys identified a route through the rugged mountains of Colorado, the state is criss-crossed with railroads to access mineral resources.  There are also geologic features that create obstacles, such as steep canyons that can make a watercourse difficult to follow. 

 

A continental divide is not necessarily the highest place on the continent.  Imagine a typical double-sloped roof with a chimney.  The roof apex is clearly the drainage divide of the roof, and this is true even if the there is a chimney on one side that is higher than the roof apex.  Drainage will just go around the chimney, and all runoff from the chimney will drain to one side of the apex drainage divide.  If you had to build a railroad across this roof, you’d build over the apex but go around the chimney – following the drainage.  The Sierra Nevada in California has higher elevations than any place on the continental divide, but is entirely on the Pacific Ocean side of the continental divide.  In fact, the continental divide (roof apex) in southern New Mexico is as low as 4,000 feet above sea level, which is 10,000 feet lower than the 14,000-foot Sierra Nevada (chimney).

 

STREAM ANTECENCE AND CAPTURE

Two competing geomorphologic processes shape the topographic surface of the earth: tectonics and erosion.  Tectonics builds and erosion wears down. If the earth had tectonics with no water erosion, all surface transportation, including railroads, would be much more difficult, for two reasons.  The first reason that water erosion facilitates surface transportation is just the general wearing down of tectonic topography, which makes mountains lower and less rugged and fills valleys and makes them flatter and thus easier to cross than without erosion.  The second reason water erosion facilitates surface transportation is that surface drainage is self-organizing, meaning that drainages coalesce in the downstream direction to form a “dendritic” drainage pattern.  The dendritic pattern forms on local and continental scales, creating corridors of constant grade (no summits or low points) that facilitate all forms of overland transportation.

 

A region with no tectonic activity and uniform rock types can approach a dendritic pattern, but tectonics and contrasting rock types can disrupt and complicate the pattern – thus the competing processes of tectonics and erosion.  In this battle between tectonics and erosion there are two geomorphologic processes in particular that effect surface transportation and railroads: stream capture and antecedent streams. Stream capture, or stream “piracy,” occurs when a stream drainage system is diverted from its own bed, and flows instead down the bed of a neighboring stream. Stream capture can happen in several ways. Headward erosion of one stream valley upwards into another is classic stream capture.

 

The now-dry valley of the original stream downstream of the capture point is a “wind gap.”  As an example, headward erosion of the Arkansas River in Colorado captured the upper Rio Grande, which formerly flowed southward over what is now Poncha Pass (a wind gap).  The resulting increased flow and erosion of the Arkansas River created Royal Gorge, pictured above with the Denver & Rio Grande Railroad along the riverbank.  In the second block diagram above, Poncha Pass is the dry gulley to the right of the capture point and Royal Gorge is the river that flows down and to the left of the capture point, now carrying its pre-capture flow and the flow of the captured river. The "Royal Gorge Route" along the Arkansas River was by far the easiest route to the continental divide in Colorado, which is why the Denver & Rio Grande and the Atchison, Topeka & Santa Fe fought for the Royal Gorge Route.  The other railroads that subsequently crossed the Colorado Rockies, including the Denver, South Park & Pacific and the Colorado Midland, had to follow multiple drainages with multiple grade summits and have since been abandoned. 

 

In addition to headward erosion, stream capture can also occur where tectonics forms a closed basin and thus blocks previous drainage patterns.  Enclosed basins form terminal lakes rather than flowing to the sea, but lakes are always temporary features.  The enclosing drainage divide wears down by erosion and, especially during wet climatic times, the basin fills with water and ultimately breaches the low point on the enclosing drainage divide.  Water flows out of the lake to an adjoining drainage basin, the flow erodes and lowers the breach even more, causing more flow and more erosion and ultimately the complete draining of the lake and the beginning of drainage organization to a dendritic pattern as basins connect.  Such was the origin of Afton Canyon used by the San Pedro, Los Angeles & Salt Lake Railroad in the Mojave Desert of southern California, pictured below.

 

An antecedent stream is one that maintains its original course despite the changes in underlying rock topography.  The erosion of an antecedent stream is sufficient to maintain its course during tectonic uplift to form a steep-walled gorge or “water gap.” 

 

Water gaps and wind gaps create routes for trails, roads, and railroads.  In the above photograph, the Central Pacific and Western Pacific railroads (now both Union Pacific) follow the Humboldt River (in foreground and in distant canyon) through a narrow gorge where the river has cut through a north-south-trending normal-fault mountain range and created a water gap that divides the range into the Tuscarora and Cortez ranges.