A Flow-Through System

December 26, 2015.  In a repeat performance from last year, the swamp is experiencing Christmas flooding. Today the Congaree River gauge at the park is reading nearly eighteen feet and has been sixteen feet or higher since December 23rd. With more water on the way down the Broad and Saluda Rivers, the park could stay flooded for the rest of the month and into the New Year. The Cedar Creek gauge is nearly 10.75 feet, about three feet above its banks. With this much water, the entire floodplain is inundated, an impressive event known as sheet flow. With sheet flow the floodplain becomes part of the river, so that in effect the Congaree River is three- and-a-half miles wide rather than the usual 400 feet when the river is confined between its banks.

Sheet flow in the floodplain from a big Congaree flood event. This is at the South Cedar Creek canoe launch.

A heavy flood of this magnitude is a reminder that the floodplain belongs to the river and the river belongs to the floodplain – the two are inseparable. It also emphasizes that, unlike most parks, the Congaree is an “open system,” a flow-through system whereby events taking place in its upper watershed a hundred and fifty miles away can have consequences much farther downstream.

Watersheds are shaped liked funnels or like trees with large, branching canopies; for the 8,000 square mile Congaree watershed, the park is located at the bottom of the funnel or the base of the tree. Much of the rainfall and runoff that falls within that watershed eventually find its way into the Congaree River, and some will end up in the park’s waterways and filter into its soil and sediments. This runoff can sometimes carry a variety of pollutants, excess nutrients, and large sediment loads, all of which can have negative consequences for the park’s environment.  The other side of that coin, however, is that the park needs floodwaters to properly function. As with just about everything else, finding that elusive balance or middle ground is the key.

Big floods, like this one from January, 2014, can put parts of the boardwalk under water. 

From strictly a layman’s viewpoint, I would say that excess sedimentation is one of the biggest threats to Congaree’s hydrologic integrity. Many of the park’s sloughs, pond, guts, and lakes show obvious signs of filling, a natural process but one that has been greatly accelerated over the past two hundred years. In fact, geology researchers have found an increase of up to 2000% in sedimentation rates at the park since early European contact, resulting in tree root collars being covered with more than six feet of excess sediment in some areas of the park!

Two hundred years of excess soil deposition on the Congaree River is revealed as a four-foot layer of red clay on top.

Heavy deposition like this can eventually kill trees and choke out aquatic life; convert hydric plant communities to drier ones; reduce the full complement of biota in lakes and ponds; and create excessively high water temperatures.

The good news is that I suspect that the current sedimentation rate in the park has slowed considerably compared to the nineteenth century when much of the Congaree watershed was cleared for cotton cultivation. Those same cotton lands have been reverting to forest for the past century, resulting in reduced soil erosion and run-off. This may not appear to be the case during a big flood on the Congaree when the river runs a rich brown from excessive sediments. Much of this now comes from urban and suburban environments, rather than farmland. Getting a handle on urban run-off will be difficult, especially with population growth increasing in the watershed, but it can be done if we put our minds to it.

The pollution picture may have improved also. It really wasn’t that long ago when the city of Columbia was dumping raw sewage, without any treatment, directly into the Congaree River. Even though recent newspaper reports raised concerns about pollution in the Congaree River and within park waters, I can’t help but believe that the Congaree is a good bit cleaner than it was forty years ago. And for me it reiterates the fact that so many of the ills that affect the Congaree National Park, as well as the larger environment itself, are human-related. This means we can also intervene to mitigate for many of these problems as long as we have the fortitude and wherewithal to so do.