See parts 1 and 2 here:
Usually a subchannel in an anastamosing river system (that is, channel other than the dominant one) or slough is correctly considered to be a former main channel that was partially abandoned or bypassed due to an avulsion (channel shift). That's the way I've treated them, at least implicitly, in parts 1 and 2. That would make them older than the main channel, which could explain why parts of them are obscured by apparently encroaching vegetation.
But what if the subchannels are actually younger than the main channel? What if they represent flow diversions (avulsions) that have persisted without eroding a distinct channel along their entire length?
This is at least conceivable because the channels often have no banks to speak of, just a transition along the channel margin from channel to wet swamp, and the floodplains and valley bottom have such low relief. Let's look at some examples. The images are derived from Michael Davias' ovoid basin survey, which is based on processing high resolution LiDAR images to identify possible Carolina Bay wetlands. Because the dataset is specifically designed to identify nearly flat, low relief features in coastal plain environment, it is also well suited for viewing fluvial and wetland landforms in the region. The imagery shown here is based on LiDAR digital elevation models with horizontal resolutions of 1 m or better.
Below is a section of the Little Pee Dee River valley. The whole valley bottom shown has no relief, other than the hummocky patterns common in alluvial swamps, and some visible channels and lakes. The lakes, which appear disconnected (and also appear so in other maps and aerial imagery), are actually connected to each other and to the Little Pee Dee at both ends (I know because I've paddled them) and convey flow at river stages well below flood levels. Furthermore, most of the valley bottom shown, though mostly forested, is nearly always inundated and conveying flow.
Section of the Little Pee Dee River valley, South Carolina. The differences in texture in the floodplain are due to different resolutions of available LiDAR data. The color legend also applies to the other images below.
Another section of the Little Pee Dee is shown below. Jordan "Lake" is actually part of a continuous flow path connected to the river on both ends, with numerous other channels and water bodies that do not show up even in this high resolution image.
Section of the Little Pee Dee River valley, South Carolina. The differences in texture in the floodplain are due to different resolutions of available LiDAR data.
Lest you think this is confined to the Little Pee Dee River, here are a couple of other examples. The section of the Black River, North Carolina shown below, like the other examples, is essentially one valley-wide flow system of channels of various size (only a few visible), floodplain depressions, and swamps that usually convey flow.
Holly Shelter Creek, a tributary of the Northeast Cape Fear River, N.C., shows some fascinating patterns, transitioning from a low sinuosity channel to a strongly meandering planform, to a complex mosaic of mostly indiscernible (on this and other images) channels to its confluence with the river.
Holly Shelter Creek, North Carolina. Arrow is the confluence with the Northeast Cape Fear River.
At all of these sites, and others like them, it is easy in a kayak or small boat to leave the main channel to explore numerous side channels, or to just paddle through the swamp forest. It is also very easy to get lost while doing so, though usually following the visible flow will get you back to the main channel.
We'll now explore several scenarios as to these channels and flow paths arise. One is a variation of the standard avulsion-and-anastamosis scenario discussed in parts 1 and 2. Flow diversions to trigger avulsions should be relatively easy, as there are no or low banks, so a crevasse is not required. A logjam--not uncommon in these systems--or simply overflow of the low banks is all it takes to deflect flow from an existing channel. Because of the low sediment loads, few of the new channels infill, and thus nearly all persist.
A variation on this theme is simply overflow through the swamp forest. Overbank flow in these systems is quite common. In this scenario there is initially un- or poorly concentrated flow through the swamp forest. Gradient and resistance selection (here I can plug my Abiotic Selection in Earth Surface Systems book) will inevitably cause the flow to become more concentrated as the flow preferentially follows locally steeper (admittedly a subtle thing in this environment) and lower-elevation paths, and avoids blockages from vegetation and woody debris. Though not to the extent in steeper settings with less vegetation, these selected flow paths tend to be self-reinforcing, via basal erosion, removal of woody debris, and inhibition of tree establishment via substrate instability and removal of seeds. The wider, visible sections, often referred to as lakes or sloughs, are probably pre-existing depressions and paleochannels. Other possible origins are creation by flood scour in backswamp swales or depressions, beaver ponds, or in historically recent times, depressions associated with logging or sand mining.
Another possibility is that the secondary flow paths are reoccupations of former channels. Some avulsion regimes in alluvial rivers are dominated by channel-switching, often involving paleochannels (Phillips, 2009). The latter represent low spots in the floodplain that can create local slope advantages, and sometimes have more easily erodible substrate.
A final(?) possibility is that the channel complex is controlled or influenced by braided patterns formed during the Pleistocene. Braided channels are a characterized by multiple, intertwining channels separated by bars that are mostly un- or sparsely vegetated. Both the bars and channels are shiftier than in anastamosing systems. Sandy braided channels existed in many streams of the southeast Atlantic coastal plain during the Pleistocene under different conditions of climate, streamflow, sediment supply, and sea-level than is now the case (Leigh et al., 2004; Leigh, 2006).
Here is a brief summary of the proposed mechanisms for development of the blackwater anastamosing channels. Based on experience (both my own and from the literature) I will not be surprised if additional explanations are identified, and I will be shocked if only one explanation (whichever it is) fits all the cases, even in the Carolinas.
1. Avulsion away from subchannel. The latter was once the main channel, and an avulsion switched flow to the current dominant channel. The subchannel is persisting as an anabranch or is in the process of infilling.
2. Avulsion into subchannel. Flow diversion into the new flow path is underway.
2A. Anastamosis avulsion (formation of a new channel that reconnects with the dominant channel downstream).
2B. Relocation avulsion underway--the newer pathway will eventually become dominant.
2C. Temporary avulsion--newer flow path is infilling and will eventually infill or become isolated from the main channel except during floods.
3. Overflow through the swamp. This is the scenario described earlier where general overflow of the channel margins forms concentrated preferential flow paths downvalley.
4. Multiple channels controlled by underlying Pleistocene braided channels.
In the 4th installment of this series, I will identify some possible ways to support or falsify these hypotheses--that is, if I can figure them out.
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References
Leigh DS. 2006. Terminal Pleistocene braided to meandering transition in rivers of the southeastern USA. Catena 66: 155–160.
Leigh DS, Srivastava P, Brook GA. 2004. Late Pleistocene braided rivers of the Atlantic Coastal Plain, USA. Quaternary Science Reviews 23: 65–84.
Phillips, J.D. 2009. Avulsion regimes in southeast Texas rivers. Earth Surface Processes and Landforms 34: 75-87.
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