BLACKWATER ANASTAMOSIS: PART 2

 This continues the post started here, where it was explained that some blackwater rivers of the Carolinas (and probably elsewhere) have anastamosing patterns that cannot be explained by the typical avulsion-based conceptual model for alluvial rivers. 

When a channel splits, or bifurcates, either both of the channels (old and new) persist, or one or the other gradually infills and becomes abandoned as a slough or paleochannel (which may be reactivated during floods). If both channels persist and rejoin downstream, this is an anastomosis avulsion. If both persist but never rejoin this is a distributary avulsion (think deltas) or occasionally a watershed fragmentation avulsion. Otherwise, one channel or the other is gradually abandoned, generally becoming plugged and disconnected first at the upstream, and later at the downstream end. 

Anastamosing patterns in the delta of the Great Pee Dee River (left), South Carolina. The channel splits are a combination of distributary avulsions (e.g., see diversion at the arrow to the Waccamaw River channel) and anastamosing avulsions (e.g., Cooter Creek). 

In some cases, however, in the blackwater rivers, channels do not appear to become abandoned and disconnected. They may sometimes appear to be isolated in air photos and other imagery because of forest canopies, but water flows into and out of these features at normal flow levels; not just floods. The photo below shows water flowing from the Little Pee Dee River into a channel called Jordan Lake. On some images this connection is invisible, and on some maps no connection is shown. But as the photo shows, the flow is quite vigorous. On the date of the photo stages and discharges on the Little Pee Dee at both the upstream and downstream gages were below median levels, and way below designated flood levels. You can also see the trees and cypress knees, and in the summer the channel is almost entirely covered by the tree canopy. 

Water flowing from the Little Pee Dee River into Jordan Lake.

The image below shows that the Jordan Lake channel reconnects with the Little Pee Dee River just upstream of the U.S. Highway 378 crossing. The channel is also fed in its lower reaches by a diversion from the Tar Lake area. Multiple such features appear in the lower Little Pee Dee River area, and in other rivers (see previous post here).

Arrows show where Little Pee Dee River (dotted line) water is diverted into Jordan Lake (upper and lower arrows) and a diversion from the Jordan Lake channel to Brunson Swamp. 

Here is my proposed scenario:

1. An avulsion (diversion of flow) occurs.

2. The sediment load of the blackwater river is insufficient to plug or infill the "abandoned" channel at the upstream (or downstream) end. 

3. Some accretion and woody debris accumulation occurs, allowing vegetation to become established in the former channel, but flow through continues. 

4. The limited sediment load prevents the semi-abandoned channel from infilling, and also keeps floodplain elevations low enough to allow frequent, perhaps nearly constant, cross-floodplain flow. 

The flow path of the former channel thus persists, despite establishment of swamp vegetation.

This scenario can, I believe, explain these features. But there are other potential scenarios as well, while I'll explore in Part 3. 

Other examples from the Little Pee Dee River. Arrows show locations where water is diverted from the river into subchannels that are not always fully evident on imagery. 

Maps shown above are based on imagery from the U.S. Geological Survey National Map. 

BLACKWATER ANASTAMOSIS: PART 1

I've had a couple of paddling trips recently on the Little Pee Dee River, South Carolina, and its backwaters and side channels. Like some other blackwater areas in the coastal plain of the Carolinas, there is a mosaic of channels and floodplain swamp wetlands, all intertwined with side channels, and with most of the floodplains having water deep enough to paddle through if you can find a path between the trees--even at typical, normal water levels; not just during floods. 

Multi-channel patterns in low-gradient river reaches are usually described as anastamosing, and can generally be attributed to streams losing the ability to transport sediment as slopes decrease toward outer coastal plain and the coast. In such aggrading systems avulsions (channel shifts) occur, and often both the original and new channels remain, creating the anastamosing pattern. 

Images from the U.S. Geological Survey National Hydrography data set, which shows only channels, water bodies, and wetland areas. (A) is along the Horry/Marion County border, SC (coordinates at center 33.9133^o N, 79.2808^o W). (B) is the Black River in southeastern NC (center: 34.4064^o N, 78.1182^o W). (C) shows tributaries of the Lynches River, SC (33.8207^o N, 79.634^o W). Areas shown in A, B are about 16 km N-S by 10 km E-W; C is 4 X 10 km). In the Little Pee Dee and Black River sites there are many more channels (as well as floodplain lakes and flowing floodplains) than shown. That is probably also the case in Lake Swamp area, but I have not been there to confirm. 

But these blackwater rivers, where upland soils are mainly sandy, the watersheds are mostly forested, and the valley bottoms are largely swamp, carry very little suspended sediment (brownwater rivers such as the Great Pee Dee, Cape Fear, and Roanoke rise in the Piedmont or Appalachians and arrive in the lower coastal plain with enough suspended sediment to give the water a tan or brown color). While the blackwater rivers do move some sandy bedload, but they are not visibly aggrading. 

Google Earth^TM image showing the contrast between the brownwater Great Pee Dee and blackwater Little Pee Dee Rivers.

Another deviation from the normal anastamosing situation is the that avulsions usually begin as a crevasse, which is river flow breaking through the natural levee on the banktop. Often the water simply spreads out in the backswamp behind the natural levee, leaving a deposit called a crevasse splay. But sometimes the breakthrough flow remains concentrated and is able to erode a channel. If there is a slope advantage relative to the river (basically, some depression that is lower than the river level), a new channel can form. In the case of a relocation avulsion, the new channel becomes the main channel and the old one may gradually infill. If both the old and new channels persist and reconnect downstream, that's an anastamosing avulsion. I worked a lot on avulsions in Gulf Coastal Plain rivers back in the day; references are at the end.

A figure from Phillips, 2014 (see below) from the Neches River, Texas, where the standard model of avulsion and anastomosis based on an aggrading system and crevasses is applicable. 

In many cases the Little Pee Dee (for instance) has no levee and no real bank--even at typical, normal water levels there is a just a flooded transition from channel to swamp. No crevasses in that situation; water can overflow the channel almost everywhere. 

Channel edge of the Little Pee Dee near Conway, SC--no real bank; no levee. 

If the traditional anastamosing model does not fit these situations, how do the multichannel planforms arise?

We will start exploring this in Part 2. 

Published work by JDP on avulsions and multi-channel patterns in Gulf Coastal Plain Rivers:

Phillips, J.D., 2014. Anastamosing channels in the lower Neches River valley, Texas. Earth Surface Processes and Landforms 39:1888-1899 

Phillips, J.D. 2013. Hydrological connectivity of abandoned channel water bodies on a coastal plain river. River Research and Applications 29: 149-160. 

Phillips, J.D. 2012. Logjams and avulsions in the San Antonio River delta. Earth Surface Processes and Landforms 37: 936-950. 

Phillips, J.D. 2011. Universal and local controls of avulsions in southeast Texas rivers. Geomorphology 130: 17-28. 

Phillips, J.D. 2009. Avulsion regimes in southeast Texas rivers. Earth Surface Processes and Landforms 34: 75-87. 

STRAIGHTENING THE WACCAMAW

Look here. The upper Waccamaw River in South and North Carolina, the section not affected by tides, has reaches that are quite sinuous, interspersed with sections that are relatively straight, with no meanders.

 

Two reaches of the Waccamaw River near the North Carolina/South Carolina border near Longs, S.C.

Sometimes such differences are attributable to tectonic effects, with an abrupt transition from a strongly meandering channel on the steeper side of a fault or flexure, and a much straighter channel on the more gently sloping side. A river's power depends on its discharge (or in relative terms, velocity) and slope. When the slope is steeper the river may have more energy and power than it needs to transport whatever sediment is available, and (according to the laws of thermodynamics) that excess energy must be dissipated in some way. Where it is dissipated against the banks, there may be erosion and lateral channel migration. This frequently occurs in the form of bends and meanders, and the channel sinuosity increases. There are various explanations for how flow hydraulics get this done, but independently of that, many studies have shown that in many cases a meandering path is the most efficient way to dissipate energy. Of course, flowing water and stream channels don't care how (or even if) energy is dissipated, but selection processes operate in abiotic as well as biological systems, and tend to select for maximum efficiency. As the meandering pattern is often optimal in this respect, erosion and deposition patterns that create meandering are selected for--that is, once initiated they tend to be preserved and sometimes enhanced and repeated (like all selection, this is probabilistic and not deterministic). Science nerds can read all about it in my Abiotic Selection in Earth Surface Systems book (Springer Nature, 2025). 

 

Bald cypress trees along the Waccamaw River, N.C. 

In trying to explain this curious pattern, the first thing I looked at was slope, controlled either by tectonic features (these are present in the Carolinas coastal plain) or paleotopography inherited from the ancestral Waccamaw River, which appears to have carried more flow than the modern version. But no luck. I also looked for potential variations in erodibility of the bank materials--they exist (they always do), but do not correspond with the straight vs. meandering reaches. But recently Gerald Nanson (a geomorphologist and river scientist in Wollongong, Australia) and I were exchanging some thoughts via e-mail about river channel patterns. Gerald, along with He Qing Huang, has done some of the most innovative and iconoclastic work on this over the years. With respect to rivers in particular and science in general, Gerald and I partially agree on almost everything and totally agree on very little, which makes for interesting and productive exchanges.

I sent a few maps of the upper Waccamaw to him and described some of my observations while kayaking. He suggested that tributaries might be bringing in a large load of sand. This would take up some of the river's excess energy to transport it, thereby reducing the bank erosion necessary to produce meander bends, and making the straighter channel the more efficient way to dissipate energy. I was pretty sure this was not correct (and I am now more so), but Dr. Nanson has not had the opportunity I have to see those tributaries and the swampy, mostly forested watersheds that are not delivering much sand.

The Waccamaw is a blackwater river with little suspended sediment. Most sediment transport is as sand bedload, as seen here. 

But the basic idea that an extra input of sand could trigger a switch from a sinuous, winding path to a straighter one got me thinking about what could deliver a load of sand. The answer is river erosion of its valley walls, which are predominantly sandy soils. It turns out that the meandering sections are mainly winding through the middle of the swampy stream valley (unconfined, in fluvial jargon), while the straight sections occur where the stream is abutting a valley wall, with erosion and bank failures delivering sand. 

 

Eroding bank along the Waccamaw River valley wall.

 

The Waccamaw River is still migrating laterally against the valley walls. Note the eroding bank valley wall in the background and the sand deposition in the foreground. 

The figures below show the same areas as the first two, but with a background of relief maps (derived from the U.S. Geological Survey National Map system). You can see the correspondence quite clearly. 

 

 

Lower panel of bottom image shows another section of the Waccamaw River, upstream of Conway, S.C. on a contour map rather than shaded relief. 

This may also be an explanation of another curious feature of the area, that I call cypress flats. These are broad areas of cypress trees in standing water that do not correspond with tributary mouths or prograding meanders. At least some of them, when you thread the kayak into them and get to dry land, are adjacent to valley walls with concave morphology indicating that they were once eroding. If I am correct, their previous erosion could deliver sand to the river that could be colonized by bald cypress (Taxodium distichum). Whether cypress could get established would depend on delivery of floating cypress seeds to the deposited sand, and the sand remaining in place and above the waterline long enough for seedlings to establish. That would in turn depend on the timing and sequence of valley wall erosion and high- and low-water events. 

 

Cypress flat along the Waccamaw River

SACKETT SUCKS IT DRY

 The Supreme Court’s 2023 Sackett v. EPA decision ruled in favor of two landowners backfilling a lot containing wetlands. The decision changed the definition of the term “waters of the United States”—which is used in the Clean Water Act—to exclude wetlands without continuous surface connections to larger, navigable bodies of water. In November 2025, the Trump administration’s EPA proposed to set new rules for water regulations that may be even looser than the updated Sackett definition.

A recent scientific study found that the new rules will leave an area of wetlands unprotected that cumulatively equals the land area of Wisconsin! This terrible news for clean water, fish and wildlife habitats, and the economic and ecosystem services provided by swamps, marshes, bogs, pocosins, floodplains, and mires. 

Backwater swamp along the Northeast Cape Fear River

While the loosened definition might not result in direct dredging and filling of the deepwater swamps that I focus on here, it would allow dredging and filling to encroach ever closer to tupelo-cypress swamps and other wetlands, inflicting numerous adverse impacts. 

Sad and tragic. 

FACTORS OF SWAMP SPATIAL DISTRIBUTIONS

 Regular readers of Swamp Things (both of you) may recall a post a few months back on The Factors of Swamp Formation. In it I applied the factorial conceptual model first developed in soil science and later applied in many other fields (especially ecology) to swamps and other wetlands. The follow-up was just published in the journal Hydrology. I initially submitted it with the "factors of swamp formation" title to resonate with the well-known factors of soil formation concept. However, reviewers pointed out, correctly, that the work deals more with spatial patterns and geographical distributions than with formation. 

The abstract is below:

Abstract

A state factor model of bottomland hardwood swamp formation is applied to a lower coastal

plain river in North Carolina, U.S., to explain variations in wetland hydrological, ecological,

geomorphological, and soil characteristics. Swamps and wetlands are a function of the

interacting influences of the state factors of climate, topography, hydrology, vegetation,

fauna, soils, geomorphic setting, and time. Five classifications of swamp and related

environments were applied to the study area, with the categories present determined

based on fieldwork. For each classification, the implicit, embedded state factors were

identified from the classification scheme itself. Relevant environmental gradients for the

study area were identified, and a spatial adjacency graph for the study area was developed

for each classification. The ability of the environmental gradients to explain the spatial

complexity of the pattern was assessed using spatial adjacency graph (SAG) analysis. All

the classification criteria are associated with the proposed state factors. SAG analysis

shows overdetermination, indicating that known gradients of causal factors are sufficient to

explain the overall pattern of spatial contiguity and that single-factor models of change are

not sufficient at the local scale. Results confirm studies showing that responses to seilla-level

and other changes are spatially patchy.

The article is open access, and you can get it via the link embedded in the citation:

Phillips, J.D. 2025 The factors of swamp spatial distributions. Hydrology 12, 332. https://doi.org/10.3390/hydrology12120332

Below, just for the heck of it, a couple of pictures from a recent paddling expedition on some anabranches and lakes of the Little Pee River, South Carolina. Though it's a bit hard to pick out at first, the top one shows a swamp tupelo (Nyssa biflora) growing right up on the root crowns of two large baldcypress (Taxodium distichum). The bottom photo is one of many cool looking cypress trees in the area. 

BACKWATERS, MUCKS, & PEATS

 In honor of World Soil Day (December 5), a post about swamp soils, even though the theme this year is "Healthy Soils for Healthy Cities." 

In 1984, the great soil geomorphologist Raymond Daniels and colleagues at N.C. State University published a bulletin for the N.C. Agricultural Research Service called Soil Systems of North Carolina. In it, they noted that the Dorovan soil series, an organic muck or mucky peat found along the lowermost reaches of coastal plain rivers, likely marked the soil effects of rising sea-level gradually encroaching upstream. I took a closer look in the field, and at soil surveys from the region, and decided they were on to something. I first looked into this from the other direction, so to speak, investigating the extent to which floodplain sediments in the lower rivers is derived from the upstream, Piedmont portion of rivers draining to the Carolinas coast (spoiler for the roughly 8.3 billion people who have not been following my work for the past 35 years: very little Piedmont sediment makes it to the lower rivers). Below is a map from a 1992 paper on the subject. The soils marked as "coastal" are those organic soils. 

I revisited this in the 2020s in investigating various impacts and indicators of the upstream effects of sea-level rise on coastal plain rivers in South Carolina, North Carolina, and southeastern-most Virginia. I wanted to know how the upstream limit of the organic soils compared to that of other indicators. And more specific to the soil issue, a key question was how a river, swamp, or floodplain transitions--apparently relatively suddenly in some cases, judging from stratigraphy--from a muddy or sandy, mineral-dominated state to one that is overwhelmingly composed of organic matter?

Soil map showing transition from mineral Chastain soil series (symbol Ch) to the Dorovan mucky peat (symbols Dk, Do) along the Roanoke River near Williamston, N.C. From SoilWeb (https://casoilresource.lawr.ucdavis.edu/gmap/)

The three main soil series involved are the Dorovan, Hobonny, and Chowan series. The Dorovan and Hobonny differ only in their acidity; the Chowan series occurs where recent mineral deposition has buried the organic soil. 

Example profiles (from SoilWeb). The layers labelled with an "O" are dominantly organic material, with the "a" indicating highly decomposed material. 

An area of Hobonny soils, Thoroughfare Island, Waccamaw River near Conway, S.C.

In this study I found that the floodplain organic soils occur well within the areas influenced by coastal backwater effects. 

Maximium distance upstream from the head of the estuary of organic floodplain soils and three other indicators of coastal backwater effects on 20 rivers (from Phillips, 2024). 

I was able to work out a sequence of changes, as shown below, with the transition to organic soils highlighted. The short version is that the hydraulic effects of sea-level encroachment both inhibits downstream transport of mineral sediment, and increases the frequency and inundation of floodplains. The latter promotes the deposition of water-transported organic matter, and due to the fact that the vegetation is adapted to wetness, does not inhibit biomass production and litterfall from floodplain plants. The wet, anaerobic environment inhibits organic decomposition, and the organic material accumulates.

Organic matter and mud deposition along the Trent River, North Carolina.

Dorovan soil along the Black River, North Carolina.

The diagram below is a more detailed look at how the formation of the floodplain Histosols fits into the various changes now occurring as sea-level inches its way upstream. 

Studying soil geography and transition problems like this could shed some light on wetland evolution and development, the little-understood dynamics of fluvial-to-estuarine transition zones, and sea-level impacts on rivers. The areas characterized by Dorovan and similar soils are part of landforms and ecosystems that have very high values for wildlife habitat, flood and storm protection, water quality, and recreation. They are also highly vulnerable not only to sea-level rise, but also rampant land development, in some cases at a nearly crazed pace (see Horry County, South Carolina for instance). While the wetlands themselves may be protected from the excavators and bulldozers, the adjacent wetlands are subject to their adverse impacts on water quality, habitat and hydrological connectivity, and space for migrating in response to sea-level change. The mineral-to-muck (or peat) shift is also an example of a system transition that may represent, and shed light on, the broader study of regime shifts and tipping points.

References:

Daniels RB, Kleiss HJ, Buol SW, Byrd HJ, Phillips JA. 1984. Soil systems in North Carolina. Raleigh (NC): North Carolina Agricultural Research Service. Bulletin 467. 77 p.

Phillips, J.D. 1992. The source of alluvium in large rivers of the lower Coastal Plain of North Carolina. Catena 19: 59-75. 

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

VITAL SIGNS

 I have reported in several previous posts about the effects of climate change on bottomland hardwood swamps of the southeastern U.S., including sea-level rise (SLR1, SLR2, SLR3), fire regimes, and  storms.

Swamps, like this one along the Waccamaw River, S.C. are at risk from climate change--just like the rest of the planet.

The Sixth State of the Climate report was recently released, and it makes for scary reading. The first paragraph is: 

We are hurtling toward climate chaos. The planet's vital signs are flashing red. The consequences of human-driven alterations of the climate are no longer future threats but are here now. This unfolding emergency stems from failed foresight, political inaction, unsustainable economic systems, and misinformation. Almost every corner of the biosphere is reeling from intensifying heat, storms, floods, droughts, or fires. The window to prevent the worst outcomes is rapidly closing. In early 2025, the World Meteorological Organization reported that 2024 was the hottest year on record (WMO 2025a). This was likely hotter than the peak of the last interglacial, roughly 125,000 years ago (Gulev et al. 2021, Kaufman and McKay 2022). Rising levels of greenhouse gases remain the driving force behind this escalation. These recent developments emphasize the extreme insufficiency of global efforts to reduce greenhouse gas emissions and mark the beginning of a grim new chapter for life on Earth.

The report, subtitled "A Planet on the Brink", is available here and a news article about it from the American Geophysical Union is titled "Our Planet's Vital Signs are Crashing."

Meanwhile, in MAGAworld and Trumpistan . . . .