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 GoogleEarthTM 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.
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. (GoogleEarthTM).
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.
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