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.
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