Meander Formation and Features of Meandering Streams
||Formation of natural levees by spill-over of sediment during floods.
Next to the channel mostly sand is deposited (highest flow velocities), and sand
compacts less than the mud that is deposited farther away. Thus, over time these
near-channel sand deposits will over time rise above the (more compacted) floodplain and
form natural levees.
||Overview of features associated with meandering streams. A
meandering stream migrates laterally by sediment erosion on the outside of the meander
(that is part of the friction work), and deposition on the inside (helicoidal flow,
deceleration, channel lag, point bar sequence, fining upwards). Adjacent to the
channel levee deposits build up, and gradually raise up the river over the floodplain
(mainly fine sediments). If the climate is humid the floodplain area beyond the levees may
be covered with water most of the time, and may form a swamp (backswamp). Rivers
that want to enter the main stream may not make it up the levee, and empty either into the
backswamp (filing it up gradually) or flow parallel to the stream for a long distance
until they finally join (yazoo streams). Meanders may cut into each other as they
grow (neck cutoffs), and then the river shortcuts (growing meanders reduce the
slope, cutoffs are a means to increase the slope again, feedback loop) and the old meander
is abandoned and slowly fills with fine sediment during floods (oxbow lakes). Also,
as a river builds up its levees and raises itself over the floodplain, the slope and the
transport power of the stream decrease, the channel fills gradually with sediment, and
finally (often during a flood) the river will breach its levee (this process is called avulsion)
and follow a steeper path down the valley.
||How meanders grow laterally through erosion (outside bend) and
sediment deposition (inside bend, point bar). When the loops get too large and
consume too much energy (friction), the river will eventually find a less energetically
"taxing" shortcut, and a part of the old channel will be abandoned and becomes
an oxbow lake.
|Air photos of meandering streams. These photos show numerous
abandoned channel segements or oxbow lakes. Oxbow lakes that are filled up with
sediments still show up on photos through different vegetation patterns (soil differences)
and are called "meander scars".