RAVINE SWAMPS IN THE MULTIVERSE

Any landscape, ecosystem, hydrological system, etc., in the grand scheme of things, is a historically contingent snapshot. Evolution, from genes and alleles to the entire Earth system, is ongoing. Change is constant, in climate, geology, ecology, and so on. And disturbances happen—hurricanes, floods, tornadoes, fires, earthquakes, human activities, and so forth. Each historically contingent environment represents a unique outcome of a single developmental pathway—but these are only one realization of what could or could have happened. All the possible pathways constitute an evolutionary potential space, which I call the geographical multiverse. The multiverse term plays off the idea of manifold timelines, familiar to quantum physicists, science fiction fans, and viewers of Rick and Morty. The “geographical” tag emphasizes the concern with environments that we experience on our planet, as opposed to the cosmological or subatomic scales typically associated with physicists and philosophers takes on the topic. 

I’ll leave the particulars of the arguments for other times and writings (but if you are interested, feel free to e-mail me). I’m a firm believer that anyone proposing theoretical notions, at least in the environmental and Earth sciences should be able to “walk the walk” with some real-world examples, and at last we arrive at the point of this post, which leans a bit toward the scientific side, though there is not too much jargon. An empirical example of the geographical multiverse notion is presented for ravine swamps along the Neuse River estuary in Craven County, North Carolina. The area features shoreline bluffs that stand about 10 m above mean water levels. These are the valley side slopes of the Neuse River drowned by Holocene sea-level rise. They were fluvially dissected during lower sea-level stands in the Pleistocene. The bluffs are therefore interspersed with steep-sided valleys containing hardwood swamps, typically perched atop clay- and organic-rich swamp soils approximately a meter above mean low water. These are ravine swamps, dominated by water tupelo (Nyssa aquatica) and bald cypress trees (Taxodium distichum)(Figure 1).

 

Figure 1.  Flanner Beach Swamp (A) and Tadpole Creek ravine swamp (B), photographed in winter. In (C) the steep slope between adjacent upland and Tadpole Creek is shown. The uprooted trees shown were blown over during Hurricane Florence in 2018. 

The ravine swamp example was chosen because it illustrates how the multiple pathway concepts apply even to small areas, and as a convenient illustration because several different landscape system states are evident within a small area. The ravine swamps are also affected by press disturbances (principally sea-level rise) and pulse disturbances, mainly tropical and extratropical cyclones. I have previously studied the impacts of Hurricane Florence on the ravine swamps and have regularly observed them for the past decade. 

 

Figure 2 below shows the topography of the study area, with the fluvially dissected valley sides, and the tributary drainages truncated by drowning of the Neuse valley during Holocene and contemporary sea-level rise and by shoreline retreat along the Neuse estuary. There are three general types of ravine swamps. The smallest, less that 0.1 km² in drainage area, are generally seasonally and occasionally flooded, with an intermittent discharge to the estuary. Larger ravine swamps such as those shown within the box on Fig. 2, are permanently flooded, with at least some standing water even during droughts, and at least some drainage to the estuary except during droughts. Larger ravine swamps such as Otter and Dam Creeks shown on the map, maintain permanent connections to the Neuse estuary. At their mouths fluvial inflow to or backwater flooding from the Neuse may occur. Larger tributaries (generally with drainage areas >100 km²) also occur, but these are not considered ravine swamps. The focus is on two ravine swamps the middle category, Tadpole Creek and Flanner’s Beach Swamp (these are not formally or officially named; names used by locals are applied)(Figure 3). 

Figure 2. Shaded relief map of the study area derived from 10 m horizontal resolution digital elevation model data (3DEP from the U.S. Geological Survey). The boxed area is shown in Figure 3.

Figure 3. Google Earth^TM image taken in February, 2024 showing the Tadpole Creek and Flanners Beach ravine swamps. 

The ravine swamps differ in five key respects, summarized in Table 1. With respect to hydrology, they may be constantly inundated or occasionally have little or no standing water. They may be regularly flowing or predominantly impounded, with only slow (except during wet periods) or intermittent outflow. The connection to the estuary may be a single dominant channel, or a dripline, where multiple small outflow points occur. Salinity also differs—though none of the ravine swamps are regularly brackish, some experience occasional low salinity due to high water levels in or storm overwash from the estuary. The swamps also differ in the state of valley infilling, though all are infilling over Holocene time scales. Most are dominated by organic matter and fine grained (silt and clay) swamp substrates, but some (along with segments of others) are dominated by sandy substrate due to storm overwash into the swamps and/or sapping erosion of the steep valley sides. The ravine swamps may have standing water right up to the edge of the upland valley sides or may have aprons of encroaching sediment around the edges, indicating some reduction of surface area. Vegetation is primarily water tupelo (or swamp tupelo, Nyssa biflora) and bald cypress, but other hydrophytes (e.g. Salix nigra, Phragmites australis; black willow and common reed). Cypress has greater salinity tolerance than tupelo, and so tends to be more dominant where salinity incursions occur. The occasionally-seasonally flooded swamps also feature other trees such as Acer rubrum (red maple) and wetness-tolerant oaks (Quercus spp.).

 

Table 1. Summary of the major factors determining the state of ravine swamps.

A key feature of the cypress and tupelo trees is that they have quite specific hydrogeomorphic conditions required for establishment. The species are hydrochores (that is, seeds are dispersed by water), so newly colonized sites must be accessible to seed transport and deposition by water. Both species are shade-intolerant, require moist conditions, and are most competitive vis-à-vis other trees where soil saturation or high water tables are frequent. Once established, tupelo and cypress can persist and thrive under conditions of constant inundation. However, they cannot germinate underwater, and submergence of the tops of seedlings will kill them. In the ravine swamps, sediment deposits from storm overwash often provide good conditions for establishment (Figure 4A). Because germination cannot occur underwater, the pioneer trees must have become established at lower sea-levels (with lower water tables), during severe droughts, or following storms, which along with sediment deposits may supply large amounts of woody debris which could serve as germination sites. Germination on downed trees since Hurricane Florence in September, 2018 by cypress and tupelo has not been observed, however, though black willow has become established. 

 

The Neuse estuary is part of the Pamlico-Albemarle Sound estuary. Due to relatively few small inlet connections to the Atlantic Ocean, astronomical tides are minimal and water level changes are dominated by wind, as shown in Figures 4B, 4C. SW winds pile up water on the east side of Pamlico Sound against the Outer Banks, lowering water levels. NE winds have the opposite effects on water levels, and given the exposure of the study area, wave attack on the shoreline also occurs. 

Figure 4. Recently deposited sand on the margin of Tadpole Creek ravine swamp with young bald cypress trees (A). Figures (B) and (C) show the cypress headland of Tadpole Creek, from upriver during strong southwest wind (B) and from downriver during strong northeast wind (C).

A more complete analysis of effects of the 2018 Hurricane is given in Phillips (2022). One key result was a transition of Tadpole Creek from a single dominant channel connection with the estuary to a dripline outlet. In 2024 Beaver moved into the site, damming the swamp at the outer edge and raising the water level about 1 m higher than before. At Flanners Beach Swamp, a dripline connection pre-Florence was converted to a single-channel outlet across deposited sand. Reduced inundation area due to marginal sedimentation was minimal at Tadpole Creek. Large areas of the Flanners Beach swamp were converted by Florence (Figure 5), and much smaller areas at Tadpole Creek. 

Figure 5. Outer edge of Flanners Beach Swamp before Hurricane Florence (A), following deposition of about 60 cm of sand during Florence (B), and in 2021 (C), looking toward the interior. The woody debris wrack line was deposited by a midlatitude cyclonic storm (nor’easter) in 2021. Note the pines in the background, which colonized a formerly inundated area covered by deposited sand.

During the most recent lower stand of sea-level in the region, the Neuse tributaries were able to cut channels and valleys, essentially dividing the upland landscape into incised channels and valleys, unincised channels at the upper, low-order portions of the channel network, and undissected areas. The incised areas experienced truncation during sea-level rise as the Neuse River valley was inundated (Figure 6). Some of these areas became ravine swamps. 

 

Figure 6. Possible evolutionary pathways in the study area. Larger gray arrows indicate multiple pathways leading to other (non-ravine swamp) states.

The watershed area of the tributary streams is determined by the original (pre-SLR) drainage area, modified by possible reorganization during SLR, when rising base levels differentially affect the streams. The lower reaches of larger tributaries are essentially drowned embayments of the Neuse estuary. These have drainage areas >100 km² (Slocum Creek, the largest tributary nearest the study area, has a watershed area of 130 km²). Smaller tributaries generally have drainage areas <10 km² (Otter and Dam Creeks have areas of 7.3 and 1.8 km²; see Figure 2). Tadpole Creek and Flanners Beach Swamp have drainage areas of 0.414 and 0.162 km², and the seasonal ravine swamps have drainage areas less than 0.1 km².

 

Those in the ravine swamp pathway shown in Fig. 6 then vary—and as outlined above, change—according to the aspects described above and shown at the bottom of the figure. Each of those criteria are determined and modified by multiple factors, the most important of which are shown in Figure 7. 

Figure 7. Major factors influencing five key characteristics of ravine swamps. SLR indicates sea-level rise. 

Five defining characteristics with two possible conditions each produces 32 possible states, in this case, with many potential transitions among them. Tadpole Creek ravine swamp at the moment is a constantly inundated swamp fed mainly by watershed runoff, secondarily by local water table fluctuations, and rarely subject to storm surge inputs, with a dripline connection to the estuary. Occasional salinity effects occur, favoring vegetation with some salinity tolerance (but not completely excluding species with little or no tolerance). Valley infilling is dominated by organic input from on-site vegetation and storm deposits of sand, with minor inputs from valley wall sapping. There is limited area reduction at present. The vegetation is cypress-tupelo swamp, with a transition in recent years from sawgrass (Cladium jamaicense) to Phragmites australis at the outer edge. A beaver dam is currently maintaining higher water levels and reducing flow velocities, allowing the floating aquatic plant duckweed (Lemna perpusilla) to proliferate.  Flanners Beach swamp, though only about 500 m away, differs in four of the five key characteristics.

 

This unique combination of characteristics is one of many possible states that could have (or could) develop. These were recently influenced by Hurricane Florence, whose impacts were strongly influenced by specific characteristics of the local and regional environmental setting, and specific characteristics of the size, track, and speed of movement of the storm (Phillips, 2022). Another large storm, departure of the beavers, a prolonged severe drought, or fire (for example), could quickly change the system state. 

 

The bottom line is that even when we consider a single type of relatively small feature, within a small area, multiple evolutionary trajectories and outcomes* occur. Ravine swamps are at home in the geographical multiverse. 

 

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*”Outcomes” does not imply any sort of final state; just the situation observed at present. 

 

Phillips, J.D. 2022. Geomorphic impacts of Hurricane Florence on the lower Neuse River: Portents and particulars. Geomorphology 397, 108026.

POINT BARS & CYPRESS RECRUITMENT

 In a previous post I discussed how it is something of a misconception that many of our swamp rivers in the Carolinas are not actively migrating laterally. The post mentioned several indicators of growth (lateral extension) of fully vegetated point bars: shoaling, recent vegetation colonization, and a younger-to-older vegetation gradient from the water’s edge inward. 

I recently paddled the section of the Waccamaw River, S.C. shown below (for those familiar with the area, this is from Red Bluff and Star Bluff upstream). You can see the river’s high sinuosity here, and some oxbows and sloughs indicating past lateral channel changes. However, except at the left side of the photo at Star Bluff, you don’t see any sandy channel-margin or point bars, suggesting lateral stability.

Portion of the Waccamaw River in Horry County, S.C. Flow is toward the left; area shown is roughly 1.5 X 2 km (Google Earth^TM image). Note: Highway 31E shown is Old Highway 31, not the Carolina Bays Parkway highway 31. 

On the river, however, it’s a different story. The indicators of point bar accretion mentioned above are present to varying extents on most bends, which also have indicators of bank erosion (erosion scarps, undercut trees, exposed roots) on the outer bend. Point bar growth plus cutbank erosion equals lateral migration.

At least two generations of recent cypress trees on a bend interior and other recent vegetation, with older trees further in. 

It was particularly encouraging to see new bald cypress (Taxodium distichum) recruitment. The growing point bars potentially offer just the right conditions—exposed to sunlight, wet, and a good spot for the water-dispersed cypress seeds to land. Even better is that many of the bars show several generations of new cypress, indicating that the recruitment is, at least in the recent past, ongoing. 

Recently established cypress on point bars. 

Several “recruiting classes” of young cypress.

PERFECT STORMS & PERFECT SWAMPS

 A recent research article found that Hurricane Idalia in the Gulf of Mexico (August 2023) was strengthened by a river plume. An extensive riverine plume in the eastern Gulf of Mexico, extending from the Mississippi-Alabama-Florida shelf to the Straits of Florida, produced a ∼20 m thick low-salinity layer and a corresponding warm upper ocean. This created a 10–20 m thick strongly stratified barrier layer below the surface layer that suppressed vertical mixing and became a critical factor contributing to Idalia's rapid intensification under the relatively less than favorable thermal and wind field environments. In other words, though the traditional meteorological indicators of hurricane strengthening did not forecast the degree of intensification that occurred, the river plume blocked the warm ocean water, allowing it to warm further, feeding Idalia (the reference and abstract are at the bottom). 

Jing Shi and the other authors, from the University of South Florida, noted that river plumes should be considered in future studies and forecasts, but that the Idalia event was a “perfect storm.” That is, an unusual confluence of atmosphere, ocean, and onshore (i.e., extensive pre-hurricane runoff and river discharge) circumstances produced the intensification.

This immediately reminded me of my own study of the impacts of 2018’s Hurricane Florence on the Neuse River and Neuse estuary. In the Neuse area the traditional indicators of hurricane strength (category on the Saffir-Simpson scale and maximum sustained winds) were unremarkable—in fact, winds along the Neuse did not even qualify for hurricane strength. However, the storm was extraordinarily large in areal extent and very slow moving, so that high winds were present four a good four days, rather than the usual <1 day. The 4 m storm surge was far above anything seen before in the region, coupled with incoming river flows from the Neuse among the highest recorded (probably THE highest, but gages failed before the flood peak arrived). Because the slower movement and higher rainfall is consistent with predictions associated with climate change, I wanted to understand what geomorphic effects of the storm were perhaps harbingers of the future, and which were attributable to specific local characteristics of the Neuse estuary and of the storm track. 

I concluded that the large area of the storm, slow forward movement, and extreme rainfall of Florence are likely indicative of a “new normal” with respect to tropical cyclones in the region, but that the geomorphic impacts in the lower Neuse were largely determined by particulars of the Neuse estuary and Florence's storm track—another “perfect storm.” Since that article was published in 2022, by the way, I have quit using the ”new normal” for climate change impacts, as we now have a constantly moving baseline and basically NO normal. 

 

One finding relevant to our beloved swamps was that the lower Neuse upstream of the estuary mostly handled the combined impacts of a huge storm surge from downstream and massive flooding from upstream just fine. This is due to the complex network of channels, subchannels, floodplains, and other features that are able to convey, store, slow, and exchange water as needed—yet another great reason to protect and preserve them!

 

I have spun off the perfect storm metaphor to argue that landscapes—including, of course, wetlands and swamps—are perfect in the sense that each reflects the combined, interacting influences of a set of environmental conditions (geology, hydrology, climate, soils, biogeography, topography, etc.) and history that makes them unique and idiosyncratic in some respects. 

Tupelo-dominated swamp along the Neuse River in western Craven County, N.C.

 

For example, there are some areas within the lower Neuse River that are almost entirely dominated by water tupelo (Nyssa aquatica). This is a common swamp tree, and known to dominate some stands, but usually (even when dominant) co-occurs with other species such as bald cypress (Taxodium distichum). But some areas along the Neuse and its side-channels contain almost nothing else in the overstory, canopy-tree layer. What is the perfect combination leading to this?

 

All tupelo, all the time

 

First, we can start with a regional climate and environmental setting conducive to bottomland hardwood swamps. Within that, N. aquatica (like Taxodium and swamp tupelo, N. biflora) require very specific hydrogeomorphic conditions to become established. It has to be wet but not underwater, and once the seedlings emerge they have to get tall enough fast enough to not get flooded in future high water episodes. The seeds (of all three trees) are dispersed mainly by water, so the location must be such that they can be transported in by flow. But all three species are present in the lower Neuse, and typically occur together in some proportion, so how did tupelo become so dominant?

 

The most likely answer is logging for cypress. Though some tupelo has historically been cut, cypress is more valuable and sought-after, and most cypress swamps in the southeastern U.S. have been logged at some point. If the cypress were of good commercial quality, most or all of them may have been harvested, leaving only tupelo to re-seed the site (thanks to Dr. Kimberly Meitzen of Texas State University, who studied this along the Congaree River, S.C., and found tupelo replacement of cypress due to cypress clear-cutting). 

A fine tupelo cavity tree

 

I have tried without success to learn something of the specific logging and forestry history of the lower Neuse (what tracts were cut and when). The area was indeed logged (and is still being logged at some sites), and the lumber industry has always been a mainstay of the region, though not so much so now as earlier in the 20^thcentury (speaking here of actual trees and wood, as opposed to pulp and paper). But specific land use histories are hard to find. If I found some sawed cypress stumps up in there it would be sufficient evidence to support the logging explanation, and I will look, because that’s what I like to do.

Let’s go stump hunting!

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Shi, J., Hu, C., Cannizzaro, J., et al. 2025. Intensification of Hurricane Idalia by a river plume in the eastern Gulf of Mexico. Environmental Research Letters 20, 024050.

Abstract

Hurricane Idalia formed on 26 August 2023 and three days later rapidly intensified from a Category 1 to Category 4 strength storm in less than 24 h over the west Florida shelf. On August 30, it made landfall along Florida's Big Bend area as a Category 3 hurricane. Strikingly, despite Idalia's moderate intensity and favorable vortex structure, neither upper ocean thermal energy nor environmental vertical wind shear conditions were as favorable during its intensification from Category 2 to Category 4 as earlier in its path, raising the question of what external factors contributed to its extreme intensification during this phase. Using satellite data, underwater glider observations, and numerical model outputs, this study reveals that, in addition to the 2023 marine heatwave, an extensive riverine plume in the eastern Gulf of Mexico, extending from the Mississippi-Alabama-Florida shelf to the Straits of Florida, produced a ∼20 m thick low-salinity layer (∼34–34.5 psu) and a corresponding warm upper ocean (>29 °C, ∼25–30 m thick). This defined a 10–20 m thick strongly stratified barrier layer below the surface layer with buoyancy frequencies exceeding 10⁻³ s⁻¹ that suppresses vertical mixing and became a critical factor contributing to Idalia's rapid intensification under the relatively less than favorable thermal and wind field environments. Therefore, incorporating the river plume in future forecast models appears to be essential to improve the accuracy of intensity predictions, especially in the areas affected by the plume, where stratification plays an important role in the intensification dynamics.

 

Phillips, J.D. 2022. Geomorphic impacts of Hurricane Florence on the lower Neuse River: Portents and particulars. Geomorphology 397, 108026.

Abstract

 

In September 2018 Hurricane Florence had severe impacts on the lower Neuse River and Neuse estuary, NorthCarolina, despite the fact that it was a minor storm in terms of traditional indicators of storm intensity. Thestorm was consistent with recent trends and predictions of tropical cyclone activity driven by Anthropocene climate warming. However, its impacts in the Neuse area were also conditioned by idiosyncratic aspects of the geographic setting and the synoptic situation. Geomorphic changes examined here include erosion of estuarine shoreline bluffs, geomorphic transformations of small freshwater swamps, and effects on the river and floodplain upstream of the estuary. The shoreline changes caused by Florence were unique with respect to previous tropical cyclones and ongoing episodic erosion, due to the extraordinarily high and unusually long duration of storm surge. Transformations of the“ravine swamps”—mainly associated with deposition of >0.6 m of sand on organic muck and open water surfaces—were similarly unprecedented. Despite high river discharges (third highest on record) and the high storm surge, fluvial impacts in the lower river and fluvial-estuarine transition zone were minimal. This is attributable to the morphology of the channel-floodplain system, adapted to Holocene sealevel rise and preserved by wetlands protection programs. The large area of the storm, slow forward movement, and extreme rainfall of Florence are likely indicative of a“new normal” with respect to tropical cyclones in the region. However, the geomorphic impacts in the lower Neuse were largely determined by particulars of the Neuse estuary and Florence's storm track. An exception is the limited impacts on the lower fluvial portion of the river and the fluvial-estuarine transition zone, where there exists a complex mosaic of channels and flowing wetlands capable of accommodating extreme discharges.

LATERAL MOVEMENT

 Swamp-flanked coastal plain rivers not uncommonly look like the pictures below, from the lower Sabine River along the Louisiana/Texas border. The river not only meanders, but the meanders and associated lateral migration are clearly active. This is evident from the minimally or unvegetated sandy point bars on the bend interiors and the active cutbanks on the outside of the bends.

Aerial and ground-level views of meander bends on the Sabine River, Louisiana/Texas

However, even in the Sabine River, bends in the lowermost reaches of the river appear to be stabilized, with a full vegetation cover and fine-grained sediment rather than sand. Cutbanks exist but are less common and spectacular than some of those upstream. This is typical of many—not all—of the swamp rivers of the Carolinas coastal plain. These are often thought to be laterally stable—that is, not migrating side-to-side—because they don’t look anything like those pictures above. This post continues the theme in this earlier post on the varied and unusual channel patterns of our Carolina swamp rivers. Here we explore some evidence that these rivers are perhaps not as laterally stable and fixed as it seems. 

A meander bend on the lower Neuse River, N.C.

If you don’t have an unvegetated, mobile sandy point bar, what evidence is there that a meander bend could be growing on its inside? Three indicators I’ve seen in the field are: (1) Shoaling due to sediment accumulation at the edge of the bend interior below the vegetation line; (2) Recent vegetation colonization on the outer edge of the bend interior; and (3) a clear successional gradient (younger to older) from the river’s edge.

Shoaling on bends of the Northeast Cape Fear River, N.C.

Recent vegetation establishment on a bend in the Neuse River fluvial-estuary transition zone (top), and on the Trent River, N.C..

Vegetation gradient on a bend of the Little Pee Dee River, S.C. (GoogleEarth image). River is about 50 m wide at the arrow.

Active lateral migration should have some indication of erosion on one side of the channel (at bends, on the outer bend). These indicators include erosion scarps, slump scars, undermined trees, and exposed roots. 

Eroding banks along the Neuse River (top), Grinnell Creek (middle), and White Oak River,  N.C. 

In addition to field indicators, there are sometimes indicators of lateral movement from maps and imagery. These take the form of paleochannels, and ridges indicating former natural levees on the channel bank. 

Shaded relief map of the Neuse River valley bottom downstream of Maple Cypress Landing. The elevation profile is along the line shown, from the left (north) side of the valley to right (Figure 4 from Phillips, 2022). 

The examples below are from my recent article on tributaries and meander bends on coastal plain rivers in the Carolinas. 

Confluence of Bigham Branch and the Great Pee Dee River. Left is a slope map

derived from 10 m-resolution DEM data. Gray areas are flat. 

Cape Fear River at Frenchs Creek.

It is therefore a mistake to assume that our swamp rivers are fixed in place and laterally stable, though they move more slowly than some other alluvial rivers. In a future post we will explore why oxbows are so rare in the region. 

References: 

Phillips, J.D. 2022. Geomorphology of the fluvial-estuarine transition zone, Neuse River, North Carolina. Earth Surface Processes and Landforms 47: 2044-2061. doi: https://doi.org/10.1002/esp.5362

Phillips, J.D. 2025. River meanders, tributary junctions, and antecedent morphology. Hydrology 12, 101. https://doi.org/10.3390/hydrology12050101

To Meander or Not

 Alluvial rivers flow through and across mainly sediments deposited by the rivers themselves, like the coastal plain rivers that flow through and play host to our beloved swamps. Such rivers almost always develop bends, the most pronounced of which are called meanders or meander bends. And many reaches of our swamp rivers do meander, some quite a bit. The standard way of measuring the “bendiness” of a channel is sinuosity, which is the ratio of the distance between two points along the middle of the channel and the straight-line, crow-flying distance. As a rule of thumb, the channel is usually called meandering if the sinuosity is >1.5 (i.e., the distance from A to B along the channel is 1.5 times the straight-line distance), but sinuosities >2 and even >3 are not uncommon.

The Little Pee Dee River, South Carolina just downstream of the N.C./S.C. state line (it is called the Lumber River north of the border).

Exactly why natural channels usually meander puts us into theoretical territory I don’t want to get into here, but there are good physical reasons for it. While there is still active research and debate on the finer points, trust me that why channels meander is not a mystery to fluvial geomorphologists.

Exceptions—that is, alluvial rivers that don’t meander—are either straight (nowhere near perfectly straight in most cases, just sinuosity <1.5), or multi-channel. Straight channels, I taught my students for years, are found in situations where the river is unable, or rarely able, to erode its banks, or where a river reach is relatively young and just hasn’t had time to develop bends and curves. Some multi-channel reaches are braided, with intertwining channels where both the channels and the islands or bars between them shift rapidly and the islands usually have limited vegetation cover. These occur mainly in steeper, gravel-bed rivers and are rare, if not totally absent (I’ve never seen one), in the coastal plain. Our multi-channel rivers are called anastomosing, where the channels and islands are more permanent and the islands are vegetated. Anastamosis requires avulsions where a channel shift occurs, and both the old and new channels persist. These in turn require an aggrading system with a net accumulation of river sediment due to the inability of the flow to transport the sediment load. These are in fact most common in low-gradient streams and deltas. 

Coastal plain rivers in the Carolinas “should” therefore be meandering or anastomosing, according to conventional wisdom and experience, and many are. But many are straight, implying either non-erodible banks, or geologic youthfulness. Straight, meandering, and multichannel reaches are often found in close proximity in the same fluvial system, and the subchannels of anastomosing reaches may themselves be straight, meandering, or both.

U.S.G.S. National Hydrography Dataset from Florence County, S.C. shows meandering Lynches River and its anastomosing (and straight, and meandering) tributary, Lake Swamp.

Check the GoogleEarth^TM image below, for example, from the lower South Carolina coastal plain. The Pee Dee River channel is straight, the Bull Creek channel meanders, and the Pee Dee/Waccamaw complex as a whole is multichannel. Bull Creek carries much of the Pee Dee flow downstream, some over to the Waccamaw channel and some back to the Pee Dee channel. 

GoogleEarth^TM  image near Georgetown, S.C.

In the region numerous examples of cases like the one below exist, where the main or trunk stream is straight but its tributary is strongly meandering (or vice-versa).

Tar River and Tranter’s Creek just upstream of Washington, N.C. (GoogleEarth^TM). 

In future posts we’ll explore possible controls over the different channel patterns, including hydrological, topographic, geologic, and ecological factors. We’ll examine some possible causes of the seemingly anomalous straight reaches, and explore a curious variation—straight reaches with roughly parallel paleochannels on the floodplain, indicating either non-meandering lateral migration, or avulsions where both channels did not persist. And we’ll examine the closely related issue of why the channels, meandering and otherwise, appear to inactive (that is, little or no lateral migration), and whether those appearances are deceiving. 

MYSTERIOUS WAYS & CONNECTION SELECTION

 Recently Oxford University Press published my first book written for a general readership, as opposed to a scientific research monograph. 

The description is below, and you can get it in hardcopy or E-book directly from the publisher here. You can also get it through Amazon, and a free chapter (till May 2026) here.

In one section of the book, I discuss the advantages of high connectivity in environmental systems. “Everything is connected to everything else” is called the First Law of geography, ecology, and environmental science, but why are things so highly connected? For the full answer, read the book. For an illustration of the advantages of high connectivity, I used the lower Waccamaw River, South Carolina—thus my excuse for plugging the book in the Swamp Things blog. 

Over a period of slightly less than three years, the lower Waccamaw River experienced the three highest flows ever recorded, during major floods in October 2015, October 2016, and September 2018. In 2015 an “atmospheric river” event pumped moisture from a tropical system well to the south and sent a firehose of wet air into South Carolina, causing extreme rainfall, runoff, and river flooding. Hurricanes Matthew and Florence in 2016 and 2018 included not only extreme river discharges but also storm surges from downstream estuaries. Irrespective of these large, high-energy flows (and in contrast to the severe impacts on humans and the built environment), ecological, geomorphological, and hydrological changes were minimal. The Waccamaw took a lickin’ and kept on tickin.’

 

Myrtle Beach Sun-News photo by Jason Lee of Waccamaw River flooding in Conway, SC on 9 October 2016. Two National Guard soldiers waded through flood waters going door to door to check on residents.  

How? Why? Because of the high connectivity among hydrogeomorphic components of the system.  Having better sense than to launch my kayak into the teeth of a hurricane, I did not directly observe what was happening in the river valley during the floods, but have made many field observations since. There is also ample aerial imagery—including some during the floods made by the U.S. Geological Survey and the National Oceanic and Atmospheric Administration specifically to assess storm and flood damage. These give a reasonable picture of what was going on at a broad scale, if not a feel for exactly how water flowed through swamps at a specific location. 

Google Earth image of part of the lower Waccamaw River.

Based on field observations and imagery, I demarcated the eight types of hydrogeomorphic elements as shown below. 

 

Hydrogeomorphic elements of the lower Waccamaw channel/wetland complex (copy of Table 8.2 from Mysterious Ways). 

Observed hydrologic connections and water exchanges among hydrogeomorphic elements of the lower Waccamaw River. Entries represent fluxes from the row to the column element (copy of Table 8.3 from Mysterious Ways).

Not many blank boxes, huh? There is a high degree of interconnectivity in this system, shown diagrammatically below. Almost everything is connected to almost everything else. The exchanges among elements are in all instances two-way, with the net direction of flux depending mainly on river stages and whether they are rising or falling, but also influenced by astronomical tides, local runoff, storm surges, and wind. This enabled the Waccamaw to absorb the flood and storm surge impacts by storing water and delaying flow through wetlands, activating spillways to transport excess water, reverse flows in some components, and conduct two-way exchanges of water during rising and falling river flows. 

Connectivity graph for hydrogeomorphic elements and water exchanges in the lower Waccamaw (Figure 8.6 from Mysterious Ways). 

The book gives some additional analysis, but beyond a demonstration of the benefits of connectivity, there is an important lesson about how vital it is to preserve the complex of swamps, back-channels, and other features of these lower coastal plain rivers.