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.

 

Bank Failure & Cypress Success

 One of the most fascinating things about bald cypress, water tupelo, and swamp tupelo trees (Taxodium distichum, Nyssa aquatica, N. biflora) is that they have very specific conditions for their natural dispersal, germination, and establishment. Because of this, where the trees are established tells a story about conditions at the times the trees got started. Certainly, this is of intrinsic botanical and ecological interest, but it also provides evidence of hydrological and geomorphological conditions, and in some cases, changes.

The seeds are dispersed mainly by water. To germinate, they need to be deposit on wet 

A mass of floating water tupelo seeds on the floodplain of the Pee Dee River, S.C.

or at least moist soil. They cannot germinate underwater, and seedlings must grow tall enough to rise above any subsequent flooding. Once that happens, the trees can survive in perpetually inundated conditions, but the spot where establishment occurs must have been subaerially exposed—wet maybe, but not underwater—for at least one growing season. One of the things that particularly interests me is cases where tupelo and cypress are growing in sites that are always underwater. They can’t have been that way when the trees got started, so something changed. For example, the Google Earth^TM image below shows a 0.5 km (0.3 mile) stretch of the shoreline of the Chowan River, N.C. 

Cypress trees growing in standing water, Chowan River. 

Here cypress and a few tupelo are growing in always-flooded conditions because shoreline erosion and drowning by rising sea-level has overcome the sites where the trees germinated. You can measure and see that >100 m of shoreline retreat has occurred over the lifetime of the oldest, farthest from the modern bank trees. 

But there are cases where trees are growing in constantly inundated conditions where it is not obvious how they ever dried out long enough for trees to establish, or how once exposed sites got drowned. Again, the tree survival and growth is no surprise—cypress and tupelo can do that—but they cannot have gotten started underwater. 

Trees growing in constantly flooded conditions along the Trent River, NC. 

Where this occurs, one of several things must have happened—an extreme drought or diversion of flow away from the site long enough for trees to germinate, perhaps, or channel changes that leave a non-flooded depression that is later filled. Or, the trees germinated on a higher spot, which could have been a log or stump from a predecessor that died. This blog will investigate more of these situations in, as they say, the fullness of time.

The situation I address now is that of trees growing along, but away from, a distinct bank (in these swamps a distinct bank is not always present—many banks are very low, or even absent, with just a gradual transition from open water to deepwater swamp). Like the example above, the trees could have started on a stream bank which later eroded, and some of them probably did. But in other cases, the field evidence makes that unlikely, and therefore a puzzle. 

Trees in perpetually flooded sites along the banks of Cedar Creek, SC (top), and Tar River, NC (bottom).

Then last week, I was paddling along Holly Shelter Creek (tributary to the Northeast Cape Fear River), and I noticed something that I should have noticed a long time before—bank failures. Slumps, slides, and rotational failures along banks can dump sediment along the stream edge where a tupelo or cypress could get established before the failure material gets washed away.

A line of tupelo and cypress of apparently similar age along a section of Holly Shelter Creek that has experienced a series of bank failures. 

Older cypress resprouting from stump—note the slump scar on the bank behind it. 

Slump scars along the Trent River.

Slump sites along Island Creek, NC. 

I feel a bit foolish for not thinking of this before, because I’ve seen more than one example like the one below, which I photographed in 2012. The slump does not have to have trees already growing on it, however—in fact, unless such trees are hydrophytes such as cypress, tupelo, some willows, etc., they won’t last long with their bases underwater. All they must do is provide a substrate that stays mostly above water for a growing season.

Bank slump along the Sabine River, Louisiana. 

Fire in the Swamplands

When I went out this morning (in Myrtle Beach, SC), it looked as though a heavy fog had settled in. My eyes began to itch, and my nose quickly confirmed that the fog was actually smoke. Horry County, parts of it at least, is burning. Night before last, my son’s family had to evacuate their neighborhood (the upside was that we got some extra grandkid side, because we all crowded into our apartment for the night). 

Fire in the Carolina Forest area of Myrtle Beach

 (photo by Horry County Fire & Rescue). 

As dangerous and inconvenient and costly as it is, this does not compare to many of the recent fires in the western U.S. But what makes it relevant to the Swamp Things blog is that much of what has been burned or is burning is forested wetlands. While wetlands do occasionally dry out and burn, you don’t really associate fire and wet areas. But that is increasingly going to change. 

National Wetland Inventory (NWI) map for a portion of the Carolina Forest area recently and currently burning. The green areas are mapped wetlands; at the bottom of the figure, you can see roads from the ever-expanding subdivisions in the area. The NWI codes including “FO” are forested wetlands. The areas shown are mostly classified as seasonally flooded. 

 

Climate attribution studies have identified the climate change drivers of increased fire frequency and severity in the west (and elsewhere). Though it is too soon to make that call for the South Carolina fires burning now, it is a good bet that more and bigger fires due to climate change are in the cards for the southeastern U.S. (if you doubt that climate change is happening, you are in the wrong blog, friend). Take a look at the figure below, produced using the U.S. Geological Survey National Climate Change Viewer . The tool uses outputs of 23 different climate models from 17 different sources or agencies. For this graphic the multi-model mean predictions were used. The viewer displays results for six different scenarios. In this case results for scenarios of 1.5, and 3^o C warming were used. Changes are evaluated relative to a 1981-2010 baseline for three different periods: 2025-2049; 2050-2074; and 2075-2100. The figure below is for the Pee Dee River basin, but generally similar results show up if you analyze other areas of the region. Of course, temperatures will generally increase but so will (on average) precipitation. But what’s important for fire regimes is the balance between precipitation and evapotranspiration, which shows up in total runoff, soil moisture, and evaporation deficits (reflecting the difference between how much plant water use and evaporation would occur if moisture is always available, versus how much will actually occur). 

Month-by-month predictions for the Pee Dee River watershed (South and North Carolina) under 1.5 and 3 degree C warming scenarios (the +1.5 is basically already upon us) reflecting the net balance between precipitation inputs and evapotranspiration outputs. The solid line represents the 1981-2010 mean; the predicted values (with error bars) indicate the deviations.

 

As you can see, it will almost certainly get drier, making droughts more likely. And more drought means more fire. 

Some current outputs from the U.S. Wildland Fire Assessment System showing the dry conditions in northeastern South Carolina and the high fire risk. 

 

Fire swamps

 

For many of us of the nerd persuasion, the term “fire swamp” conjures up scenes from the classic movie ThePrincess Bride, where the fire swamp produces not only frequent and unpredictable jets of flame, but is also the home of the notorious ROUS (Rodents Of Unusual Size). But, given that seasonally flooded and hydrologically isolated wetlands can and do burn (for instance, peat fires in the pocosin shrub bog wetlands of the Carolinas are not uncommon during dry periods), what about the deepwater swamps I am mainly concerned with in this blog? These are by their nature not prone to frequent fire and are by no means fire-adapted or even fire-dependent ecosystems like some others hereabouts (for instance, longleaf pine woodlands and savannas). 

Bald cypress (Taxodium distichum), swamp tupelo (Nyssa aquatica) and water tupelo (N. aquatica) are the iconic swamp trees. All have low to very low fire resistance. However, though fire is rare, it can be important in maintaining bald cypress dominance by reducing competition from broadleaf trees. Surprisingly, Atlantic and Gulf coastal plain floodplain and riparian communities have fire return intervals of 9 to 69 years, according to the LANDFIRE model of the U.S. Forest Service, 52 to 90 percent rated as low severity (1).

 

The upshot is that while our beloved swamps on hardly on the front lines of these particular impacts of climate change (as opposed to changes in storm frequency and severity and sea level rise), they are not immune—as the smoke in my eyes when I go outside reminds me. 

 

Some video of the Carolina Forest Fire, from resident Greg Staff, via the Myrtle Beach Sun-News. 

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(1) Fire Sciences Laboratory, 2012. Information from LANDFIRE on fire regimes of Gulf and Atlantic coastal riparian and floodplain communities. In: Fire Effects Information System, U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Missoula Fire Sciences Laboratory (Producer). Available: www.fs.usda.gov/database/feis/fire_regimes/Gulf_Atlantic_coast_riparian/all.html.