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Geology of Congaree National Park

Geology of Congaree National Park

Geologic History

Congaree National Park is located in central South Carolina and protects 27,000 acres of floodplain geology and hydrology, both of which influence this unique ecosystem. Prior to the late 1800s, the southeastern United States had more than 52 million acres of floodplain forest. Today the ecosystem within Congaree National Park is the largest intact area of old-growth bottomland hardwood forest in the southeastern United States, and contains many national and state champion trees. The purpose of Congaree National Park is “to preserve and protect for the education, inspiration, and enjoyment of present and future generations an outstanding example of a near-virgin, southern hardwood forest situated in the Congaree River floodplain in Richland County, South Carolina.” Congaree National Park has been recognized as a Natural Landmark, an International Biosphere Reserve, and a Globally Important Bird Area. The topography of the park has been shaped by many processes, including the migration of two rivers, changing sea level, faults, and the forest itself.

Map of Congaree National Park
A map of Congaree National park. Various streams flow across the floodplain and the park is bounded by the Congaree River to south and the Wateree River to the east. NPS.

Congaree National Park is ~130 miles west of the Atlantic Ocean and ~150 miles southeast of the Blue Ridge Province of South Carolina, which is part of the Appalachian Mountains, which formed during the Paleozoic through multiple orogenies (mountain-building events) as tectonic plates collided. At this time the Augusta Fault, which lies under the Congaree River Valley and trends northeast-southwest, formed due to these mountain building events. Millions of years later, the supercontinent Pangea formed as all the landmasses on Earth assembled, creating a massive Himalayan-style mountain chain along the east coast of present-day North America.  

In the Triassic, about 175 million years ago, Pangea began to break apart, and as the Atlantic Ocean began to open, South Carolina became part of the Atlantic coastline. The Augusta Fault, which was a zone of weakness, was reactivated as a normal fault. A normal fault is when one block above the fault has moved downward relative to the block below. While Pangea was breaking apart, the area that is present-day South Carolina was undergoing a process called extension, which produced rift-valleys that filled with 11,000 feet of eroded sediment from the Piedmont and Blue Ridge provinces. Continental extension in the area ended ~150 million years, during which the continental margin subsided, and relative sea level rose, depositing marine sediments in the Coastal Plain.

Throughout the Upper Cretaceous, Paleocene, and Eocene sea levels fluctuated producing upper and lower delta plain and nearshore marine depositional environments. Conglomerates, coarse-grained sedimentary rocks made up of rounded fragments, and coarse sand were deposited when relative sea level fell, and rivers flowed across the Coastal Plain. Approximately 50 million years ago a major transgression occurred and carbonate and clastic sediments were deposited across the Coastal Plain. Within Congaree National Park these sediments formed the clastic, sandy deposits in the Congaree Formation. Next to the coastal highland was a marine shelf that provided quartz sand, shark teeth, and mollusks that are found within the Congaree Formation. During the Middle Eocene this region of present-day South Carolina was warm, tropical, or subtropical as evidenced by the shallow marine limestone of the Santee Limestone Formation.

In the Oligocene, there was a major global sea level fall caused by increased Antarctic glaciation. In response the area that is present-day South Carolina sea level fell, shifting the shoreline 120 miles east-southeast. Approximately 23 and 14 million years ago, sea level fell again, and sediments were eroded and spread south from the Piedmont over the Coastal Plain. During the Pliocene, sea level rose, and a nearshore environment was only a few miles from Congaree National Park.

This image shows cypress trees in the swamp of Congaree National Park
The swamp environment near the boardwalk. Cypress knees can be seen.
Credit: Carrie McIntosh/VSFS-USGS Intern

The surficial geologic units within Congaree National Park formed in non-marine environments to marginal marine (along the coast and shallowest portion of the marine shelf). The records of changing sea-level due to tectonic activity or glaciation during the Pliocene, Pleistocene, and Holocene are seen with the Middle and Lower Coastal Plain. During the Middle Pliocene and Pleistocene, fossil pollen found within Congaree National Park suggests that the environment of present-day Congaree National Park was cooler and an open savannah environment. During the Pleistocene river valley terraces began to develop. River terraces are flat surfaces with edges that developed when a river flowed at a higher elevation than the current river. The river valley terraces within Congaree National Park are composed of estuarine to fluvial sand and clay. A complex array of river valley terraces, river channels, rimswamps, alluvial fans, oxbow lakes, and bluffs can be found within Congaree National Park.

Today the national park has very little elevation change, only 20 feet in 15 miles. Even though this floodplain has little variation in elevation, it contains varied and complex topography of ridges, levees, deep-water soughs, oxbow lakes, and intermittent and permanent streams.

Cedar Creek in Congaree National Park, SC, showing old growth bottomland hardwood forest
Cedar Creek in Congaree National Park, SC, showing old growth bottomland hardwood forest.

Flooding and its effect on the ecology

The Congaree River is the southern boundary of the park, while the eastern boundary of the park is the Wateree River. Flooding in areas that are occupied by people can cause loss of life and property, but in Congaree National Park flooding is necessary to preserving its unique ecology. Dynamic floodplain processes shape the habitats that support a diversity of plants and animals within Congaree National Park. Along the Congaree River, major flood events occur on average 10 times per year, resulting in flooding of 90% of Congaree National Park. Typically, the river levels are highest flows during the late winter to early spring, and lowest river levels are during the late summer to early fall. During rain events water from these rivers flow into the floodplain, which slows the spread of flood water, distributing the flood’s energy and volume across the landscape, and bringing life sustaining nutrients to plants and animals. Wetlands are areas where water saturates the soil and have water on the surface. The wetlands of Congaree National park store flood water, allowing it to soak into the ground. Ultimately, the floodplain and wetlands together lower flood heights, recharge groundwater, and reduce erosion. Visitors to Congaree National Park can see water levels after rain events by walking on the boardwalk.    

The boardwalk in Congaree National Park when it is flooded. A sign with trail names can be seen. The water level is up to the
A boardwalk is flooded. Flooding is a common event within Congaree National Park and brings nutrients to plants and animals within the park. NPS.

Old-growth forests effects on the land

Plants and animals can influence landscapes and understanding their impact upon the land is vital for restoration of coastal wetlands and creek systems. One example is that vegetation along the banks of rivers and creeks decreases erosion. Another example is that vegetated islands develop within floodplains and  change the streamflow. The roots of old-growth trees, including bald cypress, restrict the flow of water through the floodplain and along the banks of tributaries. This in turn, influences the deposition of organic material and sediment. Congaree National Park is an ideal location for scientists to study the interactions of old-growth forests, their influence on the landscape, and how the biology and geology change due to storm events and changing climatic conditions.

 

Ongoing hydrologic research within Congaree National Park

Congaree National Park is located downstream of multiple urban and agricultural areas. The U.S. Geological Survey (USGS) and the National Park Service (NPS) Water Quality Partnership Program supports research that investigates water quality within the National Park system to better inform policy and management needs. In Congaree National Park researchers from the USGS and NPS analyzed water samples to better understand the origin of contaminants within the park. The USGS study indicated that contaminants are transported to the park from upstream sources. One example is that pharmaceutical contaminants were detected more frequently and at higher concentrations downstream from major urban areas. While many of the contaminants occurred in areas downstream from urban and agricultural areas, some in lakes are thought to originate from park visitors. All contaminants were detected levels below those that pose a risk to human health.

A flooded trail near Weston Lake. The sign that says "Weston Lake Loop, Oakridge Trail, Kingsnake Trail" can be seen.
A trail near Weston Lake that was covered in water, 2021. Credit: Carrie McIntosh/VSFS-USGS Intern

Notable Features within Congaree National Park

The Congaree River forms the southern border of the park and meanders across the floodplain. Erosion occurs at cut banks, which are located on the outside of meanders where the highest water velocity occurs. Meanwhile point bars are areas of deposition and are located where the channel’s energy decreases at the inside of the bend. Previous channel patterns can be seen within Congaree National Park.

An oxbow lake is a U-shaped free-standing body of water that forms when a meander loop of a river is cut off when the river changes direction, a process that takes place over a decade or more. Within Congaree there are several oxbow lakes, the largest of which in Congaree National Park is Weston Lake, which can be seen from the boardwalk. Weston Lake is different from many of the other oxbow lakes within Congaree National Park because it is deep (at least 21 feet) and has a gravel lakebed. Many of the other oxbow lakes within the park have shallow silt and clay bottoms. Typically,  oxbow lakes fill with sediment and become sloughs over time, also known as semi-perennial wetlands, and host cypress-tupelo forest communities. This process occurs as the abandoned channel fills with sand, then clay, and finally peat and organic-rich material. Other oxbow lakes with Congaree National Park include Dead River, Horseshoe Lake, Devil’s Elbow, and Bates Old River.

Rimswamps are places where groundwater from nearby bluffs seeps in and gathers at the surface. The most diverse plant assemblage with Congaree National Park, including the endangered Carolina bog mint, grow within rimswamps. Near the visitor's center, Muck Swamp and contains the only muck and peat deposits within Congaree National Park. These rimswamps also filter groundwater flowing into the floodplain.

An image of Weston lake, which is surrounded by trees.
An image of Weston Lake. This is the largest oxbow lake within Congaree National Park. NPS. 

References

https://www.nps.gov/articles/coastalplain.htm

https://www.nps.gov/cong/learn/news/2013-flood.htm

https://irma.nps.gov/DataStore/DownloadFile/504630

http://npshistory.com/publications/cong/nrr-2018-1665.pdf

https://www.usgs.gov/news/shedding-light-contaminants-congaree-national-park

https://www.nps.gov/cong/learn/news/upload/Boardwalk_Talk_Summer_2010_reduced.pdf