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 33.9133o N, 79.2808o W). (B) is the Black River in southeastern NC (center: 34.4064o N, 78.1182o W). (C) shows tributaries of the Lynches River, SC (33.8207o N, 79.634o 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 EarthTM 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.
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.
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