What is the first thing that comes to mind when you think of a river? If you are from the mountains, you most likely envision clear water tumbling by and maybe some trout swimming around. But if you live down along a large, flatland river like the Missouri, Ohio, or lower Colorado it is muddy water and bottom feeders that come to mind. These two types of rivers, though wildly different in appearance, have one big thing in common: day and night they are all moving sediment downstream.
As water makes is Sisyphean journey from the oceans, into the atmosphere, and down over the land, what runs off into rivers brings sediment with it: from cobbles and boulders in mountain streams to sand, silt, and clay in large rivers. Over time this continuous conveyor belt delivers, as one pioneer in sediment studies, G. K. Gilbert, calls it, the “debris” of the continental interiors to the oceans and shapes landscapes everywhere.
A river’s ability to shape landscapes is most evident during floods where orders of magnitude more sediment is moved over hours and days compared to how much is moved during low flow periods. This was evident during the wide spread floods that ravaged the Colorado Front Range in September, 2013. Just over a year ago, massive amounts of rain fell on steep mountain slopes carrying sediment from the hills into streams and eventually rivers. In steeper areas, like the narrow canyons that spill out into the Front Range, the energy in the flow was so great it moved car-sized boulders! Smaller-sized sediment lining river channels did not stand a chance and was carried downstream until the canyon rivers became prairie rivers where the slope dramatically reduces. Here, physics took over and with less energy to carry the tons and tons of sediment moving downstream most of it stopped moving and came to rest in places like Lyons (pictured below) where three to four feet deposited in some places!
Besides resulting in a major cleaning bill, what did all of this sediment do? It changed the course of the river. Instead of flowing left in some places, the river moved right, bringing flood waters to areas not originally considered part of the floodplain. Rivers change, which can be surprising to someone living near one that has not moved appreciably in a while.
River change is not as uncommon as you might think. It does not take a biblical flood (or even a 100 year flood, with a 1% chance of occurring in a given year) to move a river. A geologic study the Mississippi River drawn out in beautiful color below (active channel in white) shows a spaghetti bowl of meander scrolls denoting historic locations of the river over the years. This change is evident even over the relatively short (from a geologic standpoint) history of our country. Look anywhere at the state lines drawn along the Mississippi River and you will see meandering borders drawn down the middle of dry land formerly known as the Mississippi, which left polyps of land belonging to one state marooned on the other side of the river.
Moving downriver, we arrive at one of the great deltas of North America: the Mississippi River delta, the final resting place for so much of the sediment that bumps, rolls, and glides downstream from the peaks of the Rocky Mountains and the cornfields of the Midwest. Or at least is used to be. Locks, dams, and river engineering has essentially turned the Missouri river, a major tributary to the Mississippi and a major source of sediment from the arid west, into a string of reservoirs. Just like when the rivers left the canyon and found the prairie in Colorado, when a river flows into a reservoir, the majority of the sediment comes to rest there. Not only does this mean reservoirs have a limited life span as they fill up with sediment naturally carried by a river, it also means that the sediment never arrives to its original destination.
Two river scientists who work extensively on large rivers estimated the original, pre-dam sediment loads that reach the Gulf of Mexico from the Mississippi river and compared that to modern measurements of sediment loads (Moode and Meade, 2010), see river graphic below). They estimate that sediment loads to the Gulf from the Mississippi River have decreased by over 75% because of human influence. They attribute about half of the decrease in sediment loads to the trapping ability of the dams we have built in the Mississippi basin and the other half to river engineering practices that attempt to keep the river from moving around and make it more suitable for barge traffic. The Mississippi River of today is a far cry from the wild and unpredictable beast that Mark Twain navigated as a young man. All of this means that the Mississippi delta, America’s Wetland, receives a lot less sediment than it used to. Add that to 1,000’s of miles of canals dug for oil exploration and extraction which bring saltwater into freshwater marshes, slowly killing off the plants that hold the sediment in place, rising sea levels, and naturally subsiding land (Törnqvist et al., 2008), and you get a delta that is rapidly disappearing. As the delta subsides, so too do the highly productive wetland ecosystems, fisheries, and protection from storm surges during hurricanes. In a recent article in the New York Times Magazine, Nathanial Rich chronicles the human impacts on this vast and vanishing area and a fight to pay for its reclamation.
We’ve come a long way from clear headwater streams down to the turbid and brackish waters of the delta. I have touched on only a couple examples of the role sediment plays in rivers and why it matters to us and the environment. But there are many more issues and stories about sediment, and the rivers that move it. Please feel free to contact me with any sediment or river related questions or comments.
Meade, R. H., & Moody, J. A. (2010). Causes for the decline of suspended‐sediment discharge in the Mississippi River system, 1940–2007. Hydrological Processes, 24(1), 35-49.
Törnqvist, T. E., Wallace, D. J., Storms, J. E., Wallinga, J., Van Dam, R. L., Blaauw, M., ... & Snijders, E. M. (2008). Mississippi Delta subsidence primarily caused by compaction of Holocene strata. Nature Geoscience, 1(3), 173-176.
Boulder cascade: Fall Creek, Colorado, © Joel Sholtes 2012
Sandy River: Yampa River at Deerlodge Park, Colorado, © Joel Sholtes 2012
Canyon Flood: Andy Cross, Denver Post © 2013
Lyons Flood: R.J. Sangosti, Denver Post © 2013
Meandering Mississippi Map: Plate 22. Fisk, H. N. (1944) Geological Investigation of the alluvial valley of the Lower Mississippi River. Army Corps of Engineers Geology and Geophysics Branch. Lower and Middle Mississippi Valley Engineering Geology Mapping Program.
River Sediment Diagram: R.H. Meade and J.A. Moody (2010). © Hydrologic Sciences
Mississippi River Delta: USGS Landsat Image, 2011: http://landsat.usgs.gov/images/gallery/239_L.jpg