Meander Formation and Features of Meandering Streams


leveeform.jpg (30040 bytes) 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.
meander.jpg (37102 bytes) 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.
meanderevol.jpg (27254 bytes) 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.
meanderscars.jpg (46708 bytes) miss_meanders.jpg (54240 bytes)
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".