How to Drown a Swamp

NASA announced recently that sea-level rose in 2024 faster than expected—and the expectation was pretty fast. To quote from their press release: “Global sea level rose faster than expected in 2024, mostly because of ocean water expanding as it warms, or thermal expansion. According to a NASA-led analysis, last year’s rate of rise was 0.23 inches (0.59 centimeters) per year, compared to the expected rate of 0.17 inches (0.43 centimeters) per year. ‘The rise we saw in 2024 was higher than we expected,’ said Josh Willis, a sea level researcher at NASA’s Jet Propulsion Laboratory in Southern California. ‘Every year is a little bit different, but what’s clear is that the ocean continues to rise, and the rate of rise is getting faster and faster.’”

Observed, estimated, and predicted rates of sea-level rise (SLR) vary geographically for any given time, and according to the emissions and climate scenarios. U.S. National Oceanographic and Atmospheric Administration (NOAA) forecasts of SLR in the Carolinas region, issued in 2022, are approximately 0.3 m by 2040 and 1.5 m by 2100, according to the Intermediate High scenario. In other scenarios, estimates are 0.26 m by 2040 and 2.17 m by 2100 (Intermediate Low), and 0.32 to 2.03 m in the High scenario. These numbers are for Wilmington, N.C.; the estimates are not too different at other forecast sites in the study region (1), but slightly faster rates are forecast for Beaufort, N.C. near Cape Lookout. One study (2) found acceleration of SLR in recent years of almost 0.05 mm yr-1 at Wilmington. Recent SLR rates in the Georgia Bight (a region stretching from the Cape Fear region in N.C. down the South Carolina and Georgia coasts and into Florida) are nearly double any previous Holocene pace, and at the current rate, they estimated water level elevations relative to the 2000 datum will be 0.46 m higher by 2050 at Wilmington.

Newport River, N.C.

So what does that mean for our beloved swamps? Swamps in the fluvial-estuarine transition zone (FETZ),sometimes called tidal freshwater forested wetlands, are hotspots for hydrological, geomorphological, and ecological responses to climate change. As sea-level rises the effects will creep upstream, with a given location gradually becoming wetter and more saline. This results in formation of ghost forests and conversion of freshwater swamps to brackish marshes at the lower end of the FETZ.

The upper end of the FETZ, defined by the upstream limit of frequent backwater effects due to lunar or wind tides or storm surges, has received much less attention. I studied these dynamics on 20 rivers from those in southeastern Virginia draining to the Albemarle Sound down to the Cape Romain/Santee River in South Carolina (3).

Kingston Lake, S.C.

First, it is important to understand that backwater effects extend well upstream of what most of us think of as the tidal portions of the rivers—an average of 71 river km (44 mi) upstream of the head of the estuary, and >100 km on several rivers. The figure below shows the sequence of events as SLR effects creep up the river valley, which are primarily occupied by frequently flooded deepwater swamps.

The backwater effects at the leading edge of upstream encroachment result in higher average river stages, which can be deduced from standard hydrodynamic equations (and from intuition and common sense). This increases the frequency and duration of floodplain inundation and raises the local water table. River hydraulic slopes and velocity are also reduced, and the backwater effects sometimes block or reverse downstream flows. This reduces sediment transport capacity.  This, plus upstream displacement of the locus of deposition (the zone of the river where the flow can no longer transport its sediment load and deposits much of it), reduces fluvial sediment inputs and floodplain deposition. This inhibits natural levee development along the banks, reducing bank heights relative to the higher water levels. These factors combine to further increase the frequency and duration of inundation, resulting in frequently or semi-permanently flooded wetlands (rather than seasonally or occasionally flooded).  

Figure 7 from (3)

Meanwhile biomass production remains high in the semipermanently flooded swamps, while anaerobic conditions associated with the increased wetness retard organic decomposition rates. Plant litter produced in situ builds up, and ponding of flood waters allows transported and suspended organic matter to settle out. This produces organic-rich surficial horizons, and eventually histic epipedons and Histosols (organic-dominated soils such as mucks and peats). Concurrently, remnants of alluvial terraces (representing floodplains formed when sea and river levels were higher) become buried by the organic alluvial soils, gradually disappearing. This burial is also affected by general valley-filling associated with SLR. Finally, vegetation changes associated with water chemistry—mainly salinity—occur, eventually followed by erosion or drowning and conversion to open water. 

These changes that occur over time during coastal submergence are evident spatially as one moves from the leading edge of effects at the upstream limit of the FETZ down to the lower FETZ adjoining the head of the estuary. Like all hydrological, geomorphological, and ecological phenomena, none of the factors is solely influenced by SLR or backwater effects, or any other single factor. Fluvial discharge, groundwater, and tidal regimes, along with local variations in antecedent topography and morphology, relative SLR or coastal submergence, human impacts, land and water use histories, and ecological variables may all affect them. 

Crabtree Swamp, S.C.

As is the case for the possibility of salt and brackish marshes to “climb” or migrate upwards in response to SLR if sedimentation is sufficient, topographic slope gradients are critical. Using a simple geometric model, we can estimate how far upstream a given amount of water level rise will extend, based on the channel slope. The figure below shows that (duh) higher rates of SLR and flatter channel slopes are associated with more rapid encroachment. The figure reflects SLR rates from less that those experienced in the 20^th century to those in the higher scenarios for the rest of the 21st, and a range of slopes found in rivers of the south Atlantic coastal plain. Even the lower rates show the effects edging upstream at a rate of a few hundred meters a year. 

Figure 8 from (3)

The changes will not be visually obvious in the short run, as they chiefly involve transition from seasonally flooded to semi-permanently flooded swamps dominated by many of the same plant species. These leading-edge transitions also occur on sections of the river that are undeveloped, not widely used, and that lack gaging stations or other regular monitoring. Transitions at the lower end are more evident, as swamps turn to ghost forests to marshes—such changes are evident on some rivers from a couple of decades worth of Google EarthTM images. The changes in between are also relatively subtle over time frames of a few years, particularly when any ongoing trends are overprinted with month-to-month and year-to-year variations, along with the effects of storm and flood events. 

Implications of these changes are poorly understood, other than that they will be important. Effects of the upstream movement of the fluvial-coastal interface include water chemistry (especially salinity, conductivity, and sulfate reduction in wetlands), hydraulic slope gradients (influencing sediment transport and deposition), net flow directions and velocities, and frequency of overbank flow. Impacts on channel and floodplain morphology include bank height, channel cross-section and planform morphology, the formation and abandonment of subchannels and anabranches, watershed fragmentation, crevasse and avulsion dynamics, and floodplain sediment storage.

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            (1) NOAA Sea Level Rise Viewer: https://coast.noaa.gov/slr/

(2) Houston, J.R. 2021. Sea-level acceleration: Analysis of the world’s high-quality tide gages. J. Coast. Res. 37, 272-279.

       (3) Phillips, J.D. 2024. Sequential changes in coastal plain rivers affected by rising sea-level. Hydrology 11, 124.