Tag Archives: sediment

Mountains and Rivers

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I’m delaying the third post on deltaic physical environments (the other two are here and here) to briefly write about a map I recently came across at Big Map Blog (which you absolutely should follow, by the way).

Colton - Mountains and Rivers - 1856

George W. Colton, "Mountains and Rivers," 1856. Image courtesy of the David Rumsey Map Collection. Click for high-resolution, zoomable image.

Produced by George Colton in 1856, the map depicts, left to right, several dozen major world rivers decreasing in length and several hundred mountains increasing in size.

Be sure to click the image above for a super high-resolution, deep-zoom version.

Although the map completely abstracts individual rivers from their particular uplands, I still love the way its arrangement of features implies the massive redistribution of sediment that takes place in large watersheds.

Of course, not all the rivers shown here end in deltas. Sometimes tides, waves, sediment loads, and the physical features of both the river and the body of water into which it drains all combine to prevent land from accreting at a river mouth, forming an estuary instead.

But even though such kinds of conditions might not allow for land-building, rest assured that each river depicted on this map is diligently transporting bits and pieces of its uplands down to its mouth. The Alabama/Mobile/Tombigbee, for example, ends in an estuary rather than a delta, but it still discharges 4.5 million tons of sediment into Mobile Bay each year. And it’s one of the smaller rivers show here (in the right third of the image).

Additionally, Colton’s map documents not only the diversity of deltaic landforms around the world, but also suggests just how many deltas have in fact been historically important sites of human occupation. Moving from left to right on the map, the Mississippi, Yangtze, Ganges and Brahmaputra (“Burrampooter” according to Colton), Niger, Tigris and Euphrates, Indus, and Rhine river deltas—to name just a handful—have all supported major population centers, some of which are thousands of years old.

And finally, I’d say that the map is just great for exploring. Zoom right in and you’ll find glaciers, active volcanos, major architectural and natural landmarks, and even some traces of Alexander von Humboldt’s Latin American expedition.

There are a lot more wonderful, super high-resolution thematic maps like this one available through both Big Map Blog and David Rumsey’s Map Collection. Be sure to check both sites out for similar treasures, regardless of your field/subject of interest.

 

The Physical Environment 2: High Ground, Low Ground

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This is the second in a series of three introductory posts focused on some of the basics of deltaic physical environments. Last time, I looked at what defines a river delta as well as the timing of coastal delta formation globally around 7000-8000 years ago.

Natural levee and backswamp

A key thing to remember about deltaic landscapes is that the rivers running through them are creating, rather than eroding, land. I know I probably sound like a broken record on that front, but it’s also easy to overlook some of its implications. When we think of a typical riparian landscape, I think we often imagine a river valley, in which the highest ground is furthest from the river.

Thomas Cole - The Oxbow - 1836

A river valley. Note the high ground is furthest from the river. Image: Thomas Cole's "The View from Mount Holyoke, Northampton, Massachusetts, after a Thunderstorm," 1836 (more commonly known as "The Oxbow"). Click to enlarge.

Because of the way rivers deposit sediments in deltas, however, the highest ground is actually closest to the river. During seasonal flooding in these landscapes, sediment-laden water rises above the riverbank. As it spreads over the floodplain, that water suddenly slows, allowing particles of suspended sediment to settle and build on the landscape.

Trudeau - Plan of the City of New Orleans - 1798

Carlos Trudeau's map of New Orleans in 1798. Note that settlement hugs the Mississippi and that lands furthest from the river are marked as "cypress swamp." Click to enlarge.

Since the heaviest (and therefore largest) particles settle first, the high ground in river deltas always forms closest to the river. This embankment is called the natural levee. The further one gets from the main channel of the river, the smaller the particles of sediment that accumulate. The relatively narrow strip of high ground that constitutes the natural levee, then, begins to slope back down until it becomes a low wetland known in Louisiana as “backswamp.”

So, while deltas are typically very flat places, what little elevation there is usually hugs the river. That topography is particularly visible in the patterns of historical settlement in the Mississippi River delta.

Abbot - Approaches to New Orleans Map - 1863

Henry L. Abbot's Civil War map of "Approaches to New Orleans," 1863. Note that 65 years after Carlos Trudeau's map, settlement still hugs the high ground adjacent to the river. Click to enlarge.

 

 

 

While levees, drainage projects, and other extensive alterations to the landscape have allowed people to build homes and expand urban areas into what was once swampy low ground, places like New Orleans first emerged on the natural levee immediately along the river.

But where does all this sediment come from?

River deltas aren’t building new land and high ground from sediment plucked out of thin air. This material all comes from somewhere. And the way to figure out that “somewhere” is to look at a map of a river’s watershed. In the case of the Mississippi, you’ll notice that its waters are supplied by streams and rivers from as far west as the Rockies and as far east as the Appalachians.

Shannon1 - Mississippi River Watershed - Wikimedia Commons

Mississippi River sediments originate as far west as the Rockies and as far east as the Appalachians. Image courtesy of user "Shannon1," Wikimedia Commons. Click to enlarge.

If we think about the distinction between river valleys and river deltas as being one of eroding versus accreting landscapes, we can start to imagine that large river systems are massive sediment redistribution programs. Rain and snowmelt falling higher up in the watershed carry weathered rocks and soils from across the continent down to the mouth of the Mississippi. There, deposited as sediment, these fragments of distant landscapes build the delta. In a way, then, Deltas are the means by which ancient mountains get transformed into new shorelines.

But humans have also radically interfered with that sediment redistribution process in deltas all over the world and the Mississippi River delta is no exception. These days, most sediments are either trapped behind dams throughout the watershed or prevented from spreading over the landscape by the levees lining the river. Land-building in the Mississippi’s deltaic plain has, except in a few locations, practically ceased.

Suspended Sediments 1700 vs 1990 - USGS Circular 1133 - 1995

Sediments reaching the Mississippi River delta have declined significantly since 1700 due to dams and levees. Diagram from Meade (ed.), "Contaminants in the Mississippi River, 1987-92," 1995, pg. 18. Click to enlarge.

References

Campanella, Richard. Bienville’s Dilemma: A Historical Geography of New Orleans. Lafayette, LA: University of Louisiana at Lafayette, 2008. Also be sure to check out Campanella’s website at http://richcampanella.com/.

Gupta, Avijit (ed.). Large Rivers: Geomorphology and Management. Hoboken, NJ: Wiley, 2008.

Meade, Robert (ed.). Contaminants in the Mississippi River, 1987-92. Reston, VA: US Geological Survey, Circular 1133, 1995.