SWAMP BEAVERS

 In 1897, all native beavers (Castor canadensis) in North Carolina were gone; hunted and trapped to local extinction. In 1939, the N.C. Department of Conservation and Development got 39 beavers from Pennsylvania and released them in the Sandhills area. In 1979, after reporting on several incidents of beavers blocking drainage canals in eastern North Carolina, I wrote a column for the small daily newspaper I worked for suggesting that this was Mother Nature’s revenge for the water quality, water chemistry, and habitat damage done by artificial drainage ditches and canals in the region (it was not well received by the local agribusiness and land development establishment). By the early 1990s, scientific articles were appearing about the recovery of beaver populations in the southeastern U.S. (1).

Beaver lodges on Pinetree Creek near Vanceboro, NC (top) and on the Black River in Bladen County, NC (bottom). 

The story in South Carolina is similar—gone due to trapping and hunting by the end of the 1800s. The U.S. Fish and Wildlife Service reintroduced beavers in S.C.’s Pee Dee region in 1940. Around the same time, Georgia beavers began to recolonize the Savannah River drainage along the GA/SC border. Both populations expanded their range, according to the S.C. Department of Natural Resources, moving toward each other. Beavers are now found in all 46 S.C. counties. 

Beavers are iconic in several ways in my corners of the scientific world. Through their dam building and pond construction, they are the prototype (other than Homo sapiens, of course) ecosystem engineer—0rganisms that modify the environment to suit their own needs and affect resources and habitats for other organisms as well. They are also a prototype for the concept of the extended phenotype. An organism’s genotype is its genetic makeup, and its phenotype is how the genes are expressed in its physiology. The extended phenotype is in essence how its genes are expressed in the external environment. Beavers are also prominent in recent and ongoing efforts to use nature-based solutions in stream and wetland restoration. 

Beavers are a significant part of swamp fauna, but though their impacts are known to be significant, we don’t know a whole lot. The Classification of the Natural Communities of North Carolina (2), in the section on coastal plain floodplain forests, states:

The most poorly known natural dynamic process of floodplains is that of beavers. Beavers can dam small stream channels or may impound tributary streams or sloughs within large floodplains. Beaver ponds can raise the local water table beyond the extent of standing water. A beaver dam on an outlet slough (gut) through a natural levee can impound a large area of a complex pattern determined by microtopography. Beavers have been returning to North Carolina for several decades, after a much longer absence . . . . Little is known about their natural population dynamics, predation, disease, nor about past pond longevity and return intervals. An important question for small streams is whether all parts of a stream are suitable for pond building, so that beaver ponds appear randomly and eventually affect the whole area, or if certain favored sites are chronically ponded while others never are. In large river floodplains, only specific sites can be flooded by beaver dams; the natural levees, high ridges, and some backswamps and sloughs are not susceptible (p. 600).

 

Answering those questions is a tall order. Many beaver ponds in swamps are remote; you could not access them in anything other than a canoe or kayak or waders. Remote sensing experts can detect beaver dams and ponds from imagery, as a 1996 study on the Roanoke River, NC did (3). However, I looked at Google Earth^TM images that spanned a period on Otter Creek in Craven County, NC before, during, and after the appearance and disappearance of a beaver dam and pond (in an area near my home where I paddle several times a year). I could not detect the emergence, presence, or loss of the feature from the images due to the complex pattern of flooding that occurs independently of any beaver ponds. 

Beaver dam and pond on upper Otter Creek, a tributary of the Neuse River estuary, The dam was not present in early 2017, and present by March, 2020. It was still there in 2022, and gone by late 2024. It is not known if the pond was abandoned, washed out by a flood, or removed by property owners. 

The classification (2) includes at least 14 different descriptions of ecological, hydrological, geomorphological, and soil transitions associated with either the establishment or drainage/abandonment of beaver ponds in the N.C. Coastal Plain alone (more are described in the Piedmont and Mountain provinces). 

 

A 2015 study of sediment trapping in beaver ponds included 4 coastal plain sites in Pitt County, NC and coastal plain sites in Virginia (4). On the coastal plain, they estimated that one pond per kilometer of stream length would result in 19 million cubic meters a year of deposition in the VA and NC coastal plains. They also noted that when beaver ponds are constructed on artificially channelized streams, they often restore the natural hydrological and ecological functions of the floodplain by reconnecting the channel and floodplain. 

 

Dam on a slough of the Black River in Bladen County, NC.

I think it likely that beavers are responsible for some of what I call water savannas or water woodland; permanently flooded floodplain depressions with mostly or entirely large, older bald cypress (Taxodium distichum) and tupelo (Nyssa biflora; N. aquatica) trees. Construction of a beaver dam floods a section of floodplain forest, and most of the trees die off (or are chewed up by beavers). Mature tupelo and cypress, however, can survive constant inundation, and while not excluded from an occasional beaver chomp (see below) are not favored. In the lower coastal plain settings where I’ve observed the water savannas, sediment inputs are very low, and abandoned beaver ponds fill in only very slowly, if at all. Tupelo and cypress remain in the ponded floodplain. This is just my proposed story, mind you, but it is plausible and fits the evidence of many of my field observations. 

 

A selection of water savannas & woodlands in the lower Neuse River drainage basin.

 

 

Beavers are not believed to chew on bald cypress, and I had rarely seen evidence of it before, but on Core Creek near Cove City, NC in January there was extensive evidence of chewing on cypress knees. 

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(1) Butler, D.R. 1991. The reintroduction of the beaver into the South. Southeastern Geographer 31, 39-43. 

 

(2) Shafale, M. 2023. Classification of the Natural Communities of North Carolina. Fourth Approximation. Raleigh: N.C. Natural Heritage Program. 

(3) Townsend, P.A., Butler, D.R. 1996. Patterns of landscape use by beaver on the lower Roanoke River floodplain, North Carolina. Physical Geography 17, 253-269. 

(4) Kroes, D.E., Bason, C.W. 2015. Sediment-trapping by beaver ponds in streams of the mid-Atlantic Piedmont and Coastal Plain, USA. Southeastern Naturalist 14, 577-595. 

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.

Alligators!

Many people who learn of my penchant for kayaking in southern swamps ask “what about alligators?” I make no effort to get nose-to-nose with alligators, but I get a thrill out of seeing them. I don’t bother them, and they don’t bother me. No alligator has ever acted aggressively toward me. If they see me coming, as I get closer they will generally submerge if they are in the water or slide into the water if they are on the bank. If I surprise one, they will thrash into the water in a noisy, splashy hurry, but though they’ve startled me, they’ve never attempted to attack or intimidate me or my boat.  

The Gulf Coast in general, and Louisiana and Florida in particular, are the gator capitals of the USA. As cold-blooded reptiles they appreciate the near-tropical warmth, and the American alligator Alligator mississippiensis grows faster and larger, on average, than further north. North Carolina is the northern limit of the alligator along the Atlantic coast, though as the climate warms, they may be edging into Virginia in the Great Dismal Swamp area. 

Pixabay.com

                                                    Photo by Dave Boardman via Pixabay.com

Alligators are denizens of swamps, marshes, slow-moving coastal plain streams, and lakes. As waterfront land is devoured by development, and wetlands historically altered by dredging, filling, and artificial drainage, gator habitat has dwindled. However, human modifications of the environment have not always been detrimental.

 

In South Carolina, prime habitat was created by wetland alteration for rice cultivation, which began in Charleston around 1680. Ditch and dike construction modified natural drainage, resulting in extensive changes in wetland plant communities. Abandoned, diked rice fields began to deteriorate after the rice industry collapsed in the early 1900s, but some of the water control structures were repaired and maintained by hunters for waterfowl hunting. According to the S.C. Division of Natural Resources these impoundments host the highest alligator population densities in the state. 

 

Though far more active in warmer times, S.C. alligators may be active year-round. June is the peak time for nest construction and egg laying in the Carolinas. Nests are generally on higher ground (a relative term indeed in marshes and swamps) within about 5 m (16 ft) of the water. According to the SCDNR, in the ACE (Ashepoo, Combahee, and Edisto) basin between Charleston and Beaufort, SC nest material is typically big cordgrass (Spartina cynosuroides), which is generally consistent with what I have seen in the field, at least where big cordgrass is present. Nests are about 1.5-1.8 m (5 or 6 ft) across and about 0.5 m (20 in) tall.  The female digs a conical chamber in the center of the nest mound and lays 40-45 eggs, then adds several layers of mud and vegetation atop the egg chamber. The eggs are kept at a constant temperature by heat produced by decomposition of the nesting material. Sex of alligators is determined by nest temperatures during the middle third of embryo development, with females born when temperatures are less than 31.5^o C (89^o F). Only males hatch when temperatures are between 32.5 and 33^o C (90.5-91.4^o F), with mixed sex ratios in between. Fewer males are produced as temperatures approach 35^o C (95^o F), a temperature beyond which only females are produced. Incubation time is typically about 63-65 days but can be as long as 77 days. Hatching success averages about 70% in South Carolina. Newly hatched gators average about 24 cm (10 in) long and weigh 45-55 g (1.5-2.0 ounces). Juvenile nest-mate alligators remain together in a group called a pod or creche for up to three years (1).

 

Gator in the Waccamaw River, SC

As with humans, alligator food habits vary with age and size. Hatchlings supplement their yolk reserve with insects, crustaceans, snails, and small fish. Grownups feed on aquatic fauna and animals that venture to the water’s edge. In estuaries, they’ll often eat blue crabs, and occasionally on dead meat (carrion). At maturity, gators are apex carnivores with no natural predators (unless you count humans).  They’ll eat deer, raccoons, wading birds, semi-aquatic mammals such as beaver and nutria, and occasionally snakes and turtles. The latter apparently know when a gator is not hungry enough to crunch them, as I’ve seen turtles sunning peacefully next to alligators on the same log. 

 

Alligators do not seek to eat humans—we’re too big. Alligator attacks on humans, while rare, can occur. Gators are shy and would rather avoid human contact. But they may become aggressive if they feel threatened, if they are provoked, or if they are protecting their nests or young. Most attacks happen when humans enter alligator habitats, such as swamps, marshes, or rivers; or when alligators occupy human-created habitats such as the ever-proliferating stormwater detention and golf course ponds of the Carolinas. 

In the Carolinas alligator-infested waters occasionally freeze, and temperatures near or below freezing can be fatal. However, by staying in their dens they can survive. A study in Lake Ellis Simon near Havelock, NC in the early 1980s showed another adaptive behavior when the lake froze. Gators were observed mostly submerged, but with their snouts just above the water—in one case with the tip of the snout surrounded by, and possibly locked into, the ice. North of N.C. the cold weather prevents reproduction by restricting breeding, nesting, and hatching, but there are cases of transported alligators in more northerly locations that survived for several winters (2).

 

By the way, the oft-told tale of alligators living in the sewers of New York city is a myth. Transported alligators have been seen and caught in the city, and small dead ones have been found in the sewers, likely former pets that outlived or outgrew their welcome. But the idea that the NYC sewers host reproducing populations of large alligators is a great story, but just that—a story (3). 

 

My impression in North Carolina waters I have frequented since the late 1970s, is that alligators are far more frequent than in the 1980s and 1990s. Back in the day I very rarely ever saw one; now I see them frequently. Other paddlers, fishermen, scientists, etc. who frequent the swamps that I’ve talked to generally have the same impression. But data are scarce. If you search online for how many alligators are in N.C., several sources will tell you that there are about 1000, usually with no indication of where that number comes from (though in one case it is attributed to the state Department of Forestry and Natural Resources, a non-existent agency). That number is too low, I am confident, though it may have been applicable decades ago, when populations were still recovering. I’ve seen about 50 in my current home county alone (Craven County), and there bound to be at least a dozen I didn’t see for every gator that I laid eyes on. That gets us to 600 right there, not counting swamps in Craven County I haven’t visited yet, and at least 24 other N.C. counties where the animals are known to occur.  

 

In 2012-2013 a group from N.C. State University surveyed alligators in 25 coastal counties where gators were previously known to exist. They used multiple detection methods and some sophisticated statistics to predict alligator occupancy based on habitats and environmental variables (see figure below). The distribution was similar to that found in the only previous survey 30 years early. They did note that “there is some indication the population may have increased in certain areas” (4). 

 

In the early to mid-20^th century alligator populations nationally declined precipitously due to hunting for food, leather, and sport. They received various levels of legal protection from the early 1960s and were protected under the 1973 federal Endangered Species Act. Populations increased, and the alligator was delisted (from endangered to threatened) in 1987, and populations in the Carolinas seem to have increased since then to the point that limited permit-based hunting is allowed in the Carolinas. 

 

Though some unregulated hunting still occurs, by far the major threats to Alligator mississippiensis in the Carolinas are wetland habitat disruption and destruction, and coastal development that not only destroys and degrades habitat, but brings humans and alligators into more frequent contact—hunting and trapping of so-called “nuisance” alligators is permitted. 

Alligators are ecosystem engineers as they modify habitats via nest construction and excavation of dens, holes, and tunnels that, in addition to benefitting the gators, also provides benefits for other critters. A study in Georgetown County, SC, using game cameras identified 81 different species at alligator nests from 2016-2021; a variety of birds, mammals, reptiles, amphibians, and invertebrates. They used the nests for foraging and feeding, traveling, and other purposes, including, in the words of the study, loafing (5).

Alligator resting, appropriately enough, in a bed of alligator weed on a Waccamaw River backwater. 

Sea-level and alligators

 

Ongoing sea-level rise will increase the salinity in some alligator habitats. Alligators are primarily freshwater animals, but they can live in some low-salinity brackish areas and may be common in environments that are usually fresh but experience occasional low-salinity saltwater intrusion. They can tolerate saltier waters from a few hours (or maybe days), but do not live in marine or high-salinity estuarine environments. They are occasionally seen on ocean beaches, but this is rare enough that it is always a big deal. As a freshwater body or river reach gets saltier as sea-level rises, it would presumably come less suitable or unsuitable for alligators. As mobile creatures, however, they can readily migrate if suitable habitats are available. And therein lies the main issue.

 

As sea-level rises various swamp and marsh habitats may migrate up onto the adjacent uplands or upriver, though the extent to which this occurs depends on the slopes the landforms and ecosystems must climb. The geomorphological, hydrological, and ecological responses are more complex than a simple translation upward and inland, but there is not necessarily a net wetland habitat loss if there is room to migrate (6)

 

The key point is “if there is room to migrate.” If the adjacent land use is a housing development, golf course, or farm—as it often is—a transition to wetland will often not be allowed. Drainage, infilling, landscaping, vegetation management, and other tactics are often using to prevent or slow sea-level driven encroachment. If the wetland-upland boundary is a bulkhead, seawall, or other hard structure there is an unsurmountable barrier to migration. Even if increased wetness and vegetation change as sea-level rises are tolerated by the human overlords, alligators may not be. 

 

Within a given limited area, SLR may reduce alligator habitat due to erosion and drowning. One study examined these effects for alligators and other threatened or endangered species at the Cape Romain National Wildlife Refuge, S.C. (7). The study found that while other species are at greater risk than alligators, erosion can erase habitat. Another study examined the present and future possibility for alligators to live on the North Carolina Outer Banks barrier islands. The gator’s short-term ability to tolerate saltwater enables them to get to the islands. They found the island habitat to be suboptimal, but the sites are at the extreme northern limit of the alligator’s range. Storms (tropical and extratropical cyclones or nor’easters) are a limiting factor here likely to be made more so by climate change and SLR, as well as—of course—development by humans (8).  

 

Along most of the rivers of the Carolina coasts, the leading edge of SLR effects are in swampy bottomlands well inland of the ocean coast where the floodplains and wetlands provide a practical buffer from adjacent development. As SLR effects propagate upstream and transform habitats—my work suggests that this is occurring at a typical rate of several hundred meters per year—it will be interesting to see whether alligator populations increase (9). 

Sea-level rise projections produced in 2022 by the National Oceanic and Atmospheric Administration for several scenarios for Myrtle Beach, SC and Beaufort, NC. 

Climate change and alligators

 

Alligators and other crocodilians are quintessential tropical and subtropical humid-region species. They simply don’t establish where it is too cold or dry. The distribution of crocodilian fossils is considered a reliable indicator of past warm and humid climates by paleontologists, based on the biogeography of modern crocodilians (10). 

 

In general, it seems that warmer climates would allow alligators to expand their range if suitable habitats are available, and a 2013 article in Slate Magazine suggested that alligators have expanded their range into Virginia (11). If climate change allows more intense winter cold spells, however, this could limit expansion or cause increased alligator mortality. This is less intuitive than general warming allowing range expansion, but consider that the Arctic is warming faster than the rest of the planet (a phenomenon called Arctic amplification). This warming reduces temperature differences relative to northern hemisphere mid-latitudes. This in turn can cause the polar jet stream in the troposphere to become wavier and less stable. The stratospheric polar vortex, a band of cold air winds that circle the Arctic, can be weakened by a more irregular jet stream. A strong vortex keeps the coldest air locked up in the Arctic, but a weaker polar vortex is prone to instability and southward shifts. These shifts--polar outbreaks--can bring Arctic air to the midlatitudes, including the Carolinas, creating extremely cold weather, even as Earth warms overall (13).

 

Climate change will not turn the Carolina coastal plains into a prairie or a desert, but there is a possibility of more frequent and severe droughts. Climate models for the Carolina coastal plain under various scenarios show increasing temperatures, and various trends in precipitation—but mostly increases. However, the amount of water in soil to support plants, or running off to feed rivers and swamps is a function not only of precipitation, but also evaporation and transpiration (water used by plants). These show, in general, soil moisture storage and runoff declining, and the evaporation deficit increasing. The latter reflects the gap between potential evaporation and transpiration (essentially, the environmental “demand” for water, or how much would be evaporated and used by plants if supplies are not limited) and actual evapotranspiration. Runoff and soil moisture decreases, and evaporation deficit increases, cannot be good for gators. But whether the changes are enough to significantly influence their range and distribution is uncertain. 

Drought map for July 2024. Climate change is likely to bring more frequent and severe droughts to the Carolinas.

NOTES:

 

(1) Information from S.C. Department of Natural Resources, https://www.dnr.sc.gov/marine/mrri/acechar/speciesgallery/Reptiles/AmericanAlligator/index.html.

 

(2) Hagen, J.M., Smithson, P.C., Doerr, P.D. 1983. Behavioral response of the American alligator to freezing weather. Journal of Herpetology 17, 402-404.

 

(3) “The Truth About Alligators in the Sewers of New York.” Cory Kilgannon, New York Times, 26 Feb. 2020.

(4) Gardner, B., Garner, L.A., Cobb, D.T., Moorman, C.E. 2016. Factors affecting occupancy and abundance of American alligators at the northern extent of their range. Journal of Herpetology 50, 541-547. The earlier survey comes from a 1983 M.S. thesis from N.C. State University by T.G. O’Brien, later published as O’Brien, T. G., Doerr, P.D. 1986. Night count surveys for alligators in coastal counties of North Carolina. Journal of Herpetology 20, 444– 448. 

(5) Rainwater, T.R., Singh, R., Tuten, C.A., et al. 2024. Fauna associated with American alligator (Alligator mississippiensis) nests in coastal South Carolina, USA. Animals 14, 620. 

 

(6) This is something I have worked on over the years. For example: Phillips, J.D. 1989. Erosion and planform irregularity of an estuarine shoreline. Zeitschrift fur Geomorphologie Suppl. 73: 59-71; Phillips, J.D. 2011. Predicting modes of spatial change from state-and-transition models. Ecological Modelling 222: 475-484; Phillips, J.D., 2018. Coastal wetlands, sea-level, and the dimensions of geomorphic resilience. Geomorphology 305: 173-184; Phillips, J.D., 2018. Environmental gradients and complexity in coastal landscape response to sea level rise. Catena 169: 107-118; Phillips, J.D. 2023. Landscape change and climate attribution, with an example from estuarine marshes. Geomorphology 430: 108666; Phillips, J.D. 2024. Sequential changes in coastal plain rivers affected by rising sea-level. Hydrology 11, 124.

(7) Daniels, R.C., White, T.W., Chapman, K.K. 1993. Sea-level rise: Destruction of threatened and endangered species habitat in South Carolina. Environmental Management 17, 373-385. While the basic points of this article are still highly relevant—when habitats are lost to erosion, species suffer, and that is happening in S.C.—it would be interesting to revisit this as sea-level rise and its impacts have accelerated, and continue to do so, since the article was published. 

 

(8) Parlin, A., Dinkelacker, S., McCall, A. 2015. Do habitat characteristics influence American alligator occupancy of barrier islands in North Carolina? Southeastern Naturalist 14, 33-40. The study emphasized short-term occupation and visitation by mainland alligators as opposed to establishment of semi-permanent breeding populations. 

 

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

(10) For example, see Markwick, P.J. 1998. Fossil crocodilians as indicators of late Cretaceous and Cenozoic climates: Implications for using paleontological data in reconstructing palaeoclimate. Palaeogeography, Palaeoclimatology, Palaeoecology 137, 205-271.

 

(11) “Alligators in your backyard.” Jackson Landers, Slate Magazine, 19 February 2013. The Virginia Department of Game and Inland Fisheries confirms sightings in the southeastern part of the state, especially in the Great Dismal Swamp area along the North Carolina border. However, the department does not consider the species native to Virginia and attributes the sightings to escaped or released captive alligators. 

 

(12) Zhang, J., Tian, W., Chipperfield, M.P., et al. 2016. Persistent shift of the Arctic polar vortex towards the Eurasian continent in recent decades. Nature Climate Change 6, 1094-1099; Hamouda, M.E., Portal, A., Pasquero, C. 2024. Polar vortex disruptions by high latitude ocean warming. Geophysical Research Letters 51, e2023GL107567.