Geology of Congaree National Park, South Carolina

Congaree National Park lies on the north-shore floodplain of the Congaree River in Richland County, South Carolina, stretching northwest from where the Congaree and Wateree rivers meet. It covers about 10,633 hectares (26,276 acres) and protects the nation's largest remaining contiguous tract of southern old-growth bottomland forest.

The confluence of the Congaree (left) and Wateree (right) rivers forms the park's southeast corner, the forest in the background. NPS photo by Steven McNamara.

Geologic setting

The park sits at the inland edge of the upper Coastal Plain, where the regional geology is a thick, wedge-shaped package of rocks and loose deposits dating from the Late Cretaceous to today. Within the park, the mapped units are all young, Quaternary deposits: alluvial fans, dune sands, terrace complexes and river alluvium of the Congaree and Wateree valleys.

• Floodplain deposits, under most of the park, record the river's recent shifts and, on a broader scale, past changes in sea level.
• River terraces in the north formed during the Pleistocene from repeated layers of estuarine to river-laid sand and clay.

A floodplain shaped by water

The park's landforms, topography and soils were shaped by the past wanderings of the two rivers, earlier climate change, geologic faults, the forest itself and human activity — and these in turn govern where plants and animals live and how people use the land.

Periodic floods sweep through the bottomland forest, bringing nutrients and sediment that renew it. The water drains a 14,000-square-mile watershed reaching into North Carolina.

Although the floodplain drops only 20 feet in almost 15 miles, its surface is surprisingly varied:

• flats and ridges;
• natural and man-made levees;
• deep-water sloughs and oxbow lakes;
• intermittent and permanent streams.

Because the land is so flat, even slight changes in elevation alter how long and how often an area floods. Those subtle gradients — created by meandering channels, fallen timber and sediment — decide where each wetland community grows, together with soil texture and how long the soil stays waterlogged in the growing season. Scientists have classified 22 plant associations on the floodplain and five more on the low bluffs to the north.

Maps and inventories

Geologic maps and reports for Congaree from the Park Service's Geologic Resources Inventory, and the park's completed Soil Resources Inventory project, are available through the NPS DataStore.

Geologic map of Congaree National Park, from the Geologic Resources Inventory. Image from the National Park Service's Geodiversity Atlas page on Congaree.

Sources

Based on "NPS Geodiversity Atlas—Congaree National Park, South Carolina," National Park Service, which cites Rankin (1977), Willoughby et al. (2005) and National Park Service reports (2014, 2016); a work of the United States government in the public domain.