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.