Some Great Falls and a Very Rivery River

Don’t tell this to people from the District of Columbia, but by most measures, the Potomac River is a mediocre stream.

In the list of the longest rivers in the United States, it doesn’t even rank in the top 25 (it barely even makes the top 50). In terms of discharge—or the amount of water that flows through it—it is even less exceptional, a meager 11,000 cubic feet per second at its mouth. It would take more than fifty Potomac’s to equal one Mississippi.

And even though history tells us that Euroamericans named the body of water for a group of indigenous Americans who lived along its banks, I like to think that the foreign settlers didn’t even bother to give it a name, bungling the Greek word for river (“potami”) in an attempt to make it sound more noble:

“What sayeth thou, John Smith? Dost thou not think that river hath an aspect quite…rivery? In the tongue of the ancient philosophers, one may even call it…potomic.”

Jokes aside, by hydrologic standards, the Potomac River is nothing particularly impressive. However insignificant it may be in size, it nevertheless holds a core place in American lore because of the out-sized influence it has played in both the development of the nation and the contours of the terrain. Besides ornamenting one of the most powerful and stately cities in the history of the world, it has had a profound role in shaping human and geologic history: a conduit, a chasm, and a corrosive.  

The Potomac: A Conduit

The Potomac River and its watershed. Image source: Wikimedia Commons

The Potomac River cuts from the Allegheny Plateau of the Appalachian Mountains through the Valley and Ridge province, meanders across the Shenandoah Valley, incises through the Blue Ridge Mountains and the Piedmont before splashing momentously over the Great Falls and Little Falls into the Coastal Plain. From there it rolls past the marbled capital before sprawling into the Chesapeake Bay, rising and falling with the tide.

Harper's Ferry
Map sketch of Harper’s Ferry in the Shenandoah Valley, with topography. This map was made following the Civil War Battle at Harper’s Ferry. Image Source: Library of Congress

The course of the river has been tied to commerce and migration since well before white settlers set foot on American continent. And that makes sense: the river is older than even the mountains through which it cuts, which themselves are more wrinkled with age than most mountains in the United States. After the founding of the United States, the Potomac was the conduit between the original states and the expansive “west,” and the use of the river and its power to move mountains has been essential to the development and growth of the nation since then.

Map published in 1743 of the British and French Colonies in America. Although it is not entirely accurate, its portrayal of the Potomac River (just south of Pennsylvania) as essentially the only river to breach the “Apalachean” Mountains is telling.  Image Source: Library of Congress

While many mountain ranges are equally impassible in all directions, the unique geology of the ridge and valley province of the Appalachian Range creates a landscape that looks like the entire eastern United States was mauled by the claw of an ancient behemoth. Scarred by differential weathering caused by the thrusting of stratified geology, this area of the United States has always been characterized by the easy passage parallel to the extensive ridges and the effective inhibition of all movement across the narrow valleys and high ridges.

Topography of Appalachian region derived from satellite radar imagery. Image Source: NASA Jet Propulsion Lab

In the early days of Euroamerican settlement in North America, routes over and around mountains were important to the development of economic and political strategies in the “New World”. The Erie Canal, for example, was the second longest canal in the world when it was completed in 1825, and skirted north of the Appalachians to the Great Lakes. However, there was no water route from the eastern Atlantic Coast to the major interior waterways such as the Ohio and Mississippi Rivers.

The Potomac River and its tributaries get pretty close naturally, as they have steadily worked down the valley walls in the Appalachian Mountains into v-shaped slits that we call water gaps. Water gaps throughout the mountains became the primary means of movement to the West in the early United States, including those made by the Potomac. However, the river is characterized by shallow, rocky beds through much of its course, so travel by boat was precarious and difficult. In order to take advantage of the work the Potomac had been at for millions of years, a series of canals was attempted along the river to connect the Atlantic coast to the Ohio River Valley through the Cumberland Narrows, a water gap made by a tributary of the Potomac (not to be confused with the more lyrical Cumberland Gap on the border of Kentucky, Virginia, and Tennessee).

This was a large and important undertaking. Its importance is underscored by the fact that, directly before George Washington was president of the United States, he was president of the Potomac Company, which attempted (and failed) to build a canal to bypass the several falls along the lower Potomac. Where Washington failed, however, the Chesapeake & Ohio Canal Company succeeded. Chartered by the US government in the 1820s immediately after the completion of the Erie Canal, the C&O canal muddled through construction of the first section of the canal up to the Cumberland Narrows, an amazing achievement of engineering and determination.

The Baltimore and Ohio Railroad’s system map, as of 1961. Image Source: Wikimedia Commons

The right-of-way through the Cumberland Narrows was a lucrative route that was fervently sought after by several transportation and freight companies. The C&O had won the right-of-way for that gap, but it was not the only company to do so. The Baltimore and Ohio Railroad also had a right-of-way that along the river. This railroad was the first railroad in the United States to open for service, and one of its first routes was through the Cumberland water gap. In fact, the railroad beat the canal there, paralleling the canal’s eventual path. Even though the canal was largely successful for its first several years, the railroad eventually proved to be a much faster and cheaper way to bypass the river’s rapids. In fact, the canal was never quite what it should have: it had been planned to run all the way to the Ohio River, but the final section of the C&O Canal was never completed, and after a heyday from the 1850s-1870s it went into disrepair when the railroad was able to bring down costs. The Baltimore and Ohio Railroad eventually bought out C&O’s share of the right-of-way. “Linking 13 great states to the nation,” the Railroad (and the canal) made effective use of the river’s erosive work as they snaked their way through the constricted breaches in the great mountains. The Baltimore and Ohio Railroad—often shortened to B&O—was the only railroad memorialized in the board game Monopoly that did not directly serve Atlantic City.

This same water gap and general route also cradled the first major highway built by the federal government: The National Road a precursor to the interstate freeways ubiquitous in the country today. Even today, the cut made by the Potomac serves as one of only five passages made by freeways through the eastern mountain ranges, and even serves an extensive bike trail that showcases the beauty of the mountain ranges.

The Potomac: A Chasm

Seat of War on the Potomac
A map made during the Civil War showing the “Seat of War” in Virginia along the Potomac. Image Source: Library of Congress

As the early United States begin to grow, the Potomac served as an informal boundary between the agrarian south and the industrial north (see images below). Erroneously equated with the Mason-Dixon line, the river was the embodiment of the divides that ran beneath the veneer of the newly “united” nation. In acknowledgement of this reality, the Compromise of 1790—an agreement between Hamilton, Jefferson, and Madison that is regarded as one of the most important negotiations in American history—located the young nation’s capital on both sides of the river, originally encompassing both Alexandria, Virginia and Georgetown, Maryland in exchange for the forgiveness of states’ war debt. This may not seem like much, but it was monumental.

The country had been founded in Philadelphia. The headquarters of the war had been there, and the administrative duties of the government since then had either been headquartered in Philadelphia or in New York. When the Constitution–finally ratified in 1789–said a site should be chosen for the capital outside of the state structure, it did not specify where. So, New York and Philadelphia served as the interim capital for several years while Congress decided where to locate the new federal district. In the summer of 1789 they decided, but they did not agree: the Senate approved a site near present-day Columbia, PA and the House approved a site near Philadelphia. The Compromise of 1790 (and the attendant Residence Act of 1790) totally avoided Pennsylvania and brought the center of power down south over 100 miles from the nation’s birthplace to the tidal swamps near the head of navigation of the Potomac. An above-average stream overnight became the center of the American universe. This central role would be augmented by events yet to come.




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The Potomac was a key player in the Civil War, and during this time, its role in rivening the country came to the fore (riven is the word from which river is descended, and means “to divide”). As the map below shows, a large portion of the battles of the Civil War were fought on or within a few miles of the River that divided North and South, and with good reason: the capital of the Union was located just on the other side. During the Civil War, the Potomac served as a moat of sorts, making a campaign into the northern half of the nation quite difficult for the seceding states. The few battles that were fought on the northern side of the Mason-Dixon Line (or the Potomac River, as far as we are concerned) were often attempts by the South to veer around the river and attack Washington from the North with no Potomac as protection.

Civil War Sites
Sites of Civil War battles, with a large concentration along the Potomac River. Image Source:

As the Civil War faded into memory, the role of the Potomac as a divider has diminished. However, since ancient times and continuing today, the river has been cutting through the rock, making straight a path through the wilderness, raising up the valleys, and bringing low the mountains.

The Potomac: A Corrosive

The Potomac River at Great Falls. Image Source: Riley Balikian

As we’ve noted above, the brute resolve of the Potomac as it cascades over bedrock channels through inhospitable terrain is also responsible for much of the history that has developed around and along the body of water.The Potomac River is an old river, one of the oldest in the nation, but it formed just like all rivers form: the land surface tilted, or at least one part rose/fell faster than another. In the Appalachians, as far as geologists can tell, this process has been going on for at least 440 million years. But it wouldn’t be fair to say the Potomac started then. The many mountainbuilding episodes in geologic history since then have changed the character of the Appalachians as well as its run-off regimes. But regardless of its exact date of birth, the Potomac and its ancestral streams have worn down what had been perhaps the largest mountain range in the history of North American from “conspicuous heights” to the relatively flat range we know today. Whether Himayalan or Andean in scale (scientists aren’t sure exactly how high they were, just that they were pretty darn tall), the ancient Appalachians produced a gradient that would have thrust runoff water down, in the process tearing down the mountains as well. Much of the remains of the ancient giants has settled into the Atlantic Coastal Plain and in the valleys between mountains, but evidence of these old mountains remain. 

Scientists use natural “clocks” in the rocks exposed by the river to figure out just how erosive the water has been. Since at least 1986, scientists have used neutrons that cascade onto earth naturally from interplanetary space (“cosmogenic neutrons”) to determine how long rocks have been exposed to the surface. Cosmogenic neutrons bombard a nucleus, and convert certain elements to other elements. One radioactive element used fairly often in this type of dating is Beryllium-10. Beryllium-10 dating can help scientists understand how long a rock has been exposed to the surface, because the decay rate of Beryllium-10 is changed greatly by the presence of cosmogenic neutrons, and cosmogenic neutrons are unable to penetrate deeply into rock or water. Using this method, scientists have been able to gauge how the river has changed shape over time, and estimate just how much mountain the river has actually taken with it. The Potomac River may be small, but it is a mighty river

The Burning of Washington by the British in 1814. Image Source: Library of Congress

But even if events as diverse as ancient erosion and an American Civil War showcased the strength a river can lend, the War of 1812 revealed the weaknesses associated with clinging close to a river as well. In the War of 1812, the English–with arguably the strongest navy in the world at the time–relatively easily invaded Washington via the Potomac, sacking the city and burning many of the buildings, including the White House. For much of the war, it was uncertain if America would maintain all of its territories, or even remain intact. The attack on Washington was devastating, and was part of a series of defeats that almost spelled the end of the young nation. However, the war began to turn in the United States’ favor, and the war ended as essentially a stalemate. Regardless of the outcome of the war, the river granted direct access by one of the most powerful militaries in the history of the world to the capital of one of the youngest nations in the world. While the river had provided so much for the young nation, the British campaign on Washington was a reminder that it was far from tameable.

Winding down

The Potomac River, though seemingly insignificant in scale, has had a profound role in shaping the history and physiography of the United States. It has itself been a character in the drama of the American history, participating in the highest and lowest moments of the development of the American character. And it continues to sculpt the democracy we hold dear.

From mountains to shining sea, it tumbles from hardship to hardship, slowly and patiently digging out a path through the scarred geology and history of the continent as it transforms and chisels its way into the heights of some of the greatest mountains in the history of the world. The course of the river parallels the history of the nation in an epic struggle to create Eden from disorder. Divisive, yes, but held in high regard for its power and grit. Even when the Potomac divided the nation, it still was a life-giving stream that, like the river in New Jerusalem, nourished a capital that was designed to be a shining City on a Hill. The Potomac is, in that sense, an archetypal river.

With that in mind, maybe they wouldn’t have been so far off in calling it “rivery.”

PotomacRiver A bend in the Potomac River near Great Falls. Image Source: Riley Balikian

This post is being written from the centennial meeting of the American Geophysical Union in Washington, D.C. This conference is usually held in San Francisco, but for this special 100th meeting, it has moved to the nation’s capital. As part of this conference, I was privileged to visit the Great Falls National Park with geologists who have spent their careers studying the development of the Potomac River as it tumbles over fifty feet out of the Piedmont, into down into the Coastal Plain just north of the nation’s capital. It is a stunning location, whose magnificence is amplified by the integral part this relatively small river has played in the geologic and historic development of the region.

Feature Image: Sketch of the Civil War Showing the District of Columbia along the Potomac River. Image Source: Library of Congress

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