At 6:10 a.m. today my Weather Alert radio sprang to life. It got my immediate attention, of course, but with an extra measure of curiosity. Here in central South Carolina, the weather alerts we get in the late summer usually come in the afternoon or evening and typically announce watches or warnings associated with severe thunderstorms or tornadoes. What could this one be about?
In short order I learned that the Congaree River below Columbia hit flood stage (115.0 feet) at 6:00 a.m. and was expected to crest at 116.9 feet by around 8:00 a.m.
This is not very alarming news. Two feet over flood stage is not a major flooding episode for the Congaree, nor is there much property on the flood plain subject to damage. Developers have been smart enough to avoid the low lying terrain downriver from Columbia. A water treatment plant will be protected by its levees. Some crops in low lying fields will be inundated. Some livestock producers will have to move cattle and hogs to higher ground if they haven’t done so already. Some fishermen and deer hunters will need to adjust their plans for a few days.
In truth, some are glad to see this flood. That particularly goes for those of us who know and love Congaree National Park. Most of the park’s extraordinary forest is situated on the Congaree River flood plain, and if it weren’t for periodic floods like this one, there would be no forest of national park and International Biosphere Reserve stature in the Congaree Swamp.
The Congaree River is nicely set up for flooding. Formed by the confluence of the Broad and Saluda Rivers at Columbia, it gathers the flow of an 8,000 square-mile watershed extending all the way into the Appalachian foothills of western North Carolina and northwest South Carolina.
The Congaree is perched just below the Fall Line on the highest of a series of terrace-like structures formed when the ocean level was much higher than now. The river’s gentle gradient – only a 20-foot drop over 28 miles of river – ensures a slow, meandering flow and a wide flood plain. The Congaree is a short river, being only 20 miles long as the crow flies (and 60 miles long as the river meanders). It ends at its confluence with the Wateree River, where the Santee River begins.
Congaree National Park occupies about 40 square miles and consists mostly of river bottom hardwood forest -- sweet gum, water oak, tupelo, cypress, loblolly pine, etc. -- sprawling across thousands of acres of Congaree Swamp flood plain. (The park was originally designated Congaree Swamp National Monument in 1976.) Although the soggy ecosystem occupying the Congaree flood plain bears the name “swamp,” it is not permanently inundated and is more accurately described as a river bottom hardwood forest.
Congaree National Park is very special because it contains the most extensive remaining old-growth river bottom hardwood forest in the southeastern United States. About half of the park’s area – roughly 11,000 acres -- consists of flood plain forest that was never subjected to significant logging or other commercial exploitation.
Periodic flooding plays a key role in the ecology of flood plain forests like that of the Congaree. When the river spills out of its banks (over the natural levees) and unto the flood plain, it not only delivers moisture, but also deposits deep layers of nutrient-laden sediment on the flood plain floor. It is these rich layers of muck, peat, and related alluvial soils that encourage the growth of a lush forest with huge trees (including some state and national champions) and the tallest canopy in the Western Hemisphere (about 130 feet).
Depending on sporadic heavy rains brought on by thunderstorms, weather fronts, and occasional hurricanes and tropical storms, about 10 flooding events a year can be expected. In a typical year, about 90% of the area of Congaree National Park is inundated by flood waters at one time or another.
A drought of moderate severity has plagued this part of the Southeast for quite a while now, so we’ve had fewer floods than normal.
Flooding episodes usually last about three days, and flooding is especially common in late winter and early spring (February and March). Rains associated with hurricanes or tropical storms also cause flooding from time to time, especially from late summer through fall. The rains that triggered the current flooding were associated with the passage of tropical storm/depression Fay.
When the Congaree River floods, the waters are dispersed on the flood plain through backwaters called sloughs and small, intermittent watercourses called guts. Sloughs and guts are dotted with water tupelo and bald cypress, two tree species that flourish in standing or gently flowing water.
Heavy flooding can cause some property damage in the park (see accompanying photo), but most flooding episodes produce only minor inconveniences. While the heavily-used high boardwalk leading from the bluff to Weston Lake is perched well above the reach of most floods, the lower boardwalk trails are often completely submerged. The flood waters make some of the park’s ground trails temporarily inaccessible, too, and may leave them muddy and clogged with debris in some places. During heavy flooding, canoeists may find it difficult to follow the Cedar Creek Canoe Trail. For the most part, inconveniences like these last for only a few days.
After the flood recedes, visitors can’t help but notice the high water mark (or water line), which is discernible on the trunks of the trees where the brown stain and moss ends. This records the level of the most recent flooding.
Over thousands of years, how many marks like this have been left by how many floods in the Congaree Swamp? The mind boggles at the thought.
Thanks to that 6:10 a.m. Weather Alert announcement, I know that flood waters are creeping up the sloughs and guts and spreading through the Congaree Swamp even as I write this. This puts a smile on my face. If the flood plain forest of the Congaree were a sentient creature like you or me, it would be shouting “bring it on!”.