River deposition landforms

Alluvial fans form where streams from higher ground break out onto low-gradient foot-slope plains. The coarse load from the mountains is too heavy to carry on the gentle slope, so it is dumped as a broad, cone-shaped deposit. Streams on a fan keep shifting, forming many channels called distributaries. In humid areas fans are low, gentle cones; in arid areas, high, steep cones.

Deltas are like fans, but form where rivers dump their load into the sea. If the sea does not carry it away, it builds up as a low cone. Unlike fans, delta deposits are well sorted and clearly stratified: coarse material settles first, while silt and clay go farther out. As the delta grows, its distributaries lengthen and it builds seaward. (Example: the Krishna delta.)

Floodplains are a major landform of river deposition, just as valleys are of erosion.

• Coarse material drops first where a channel meets a gentle slope; fine sand, silt and clay are carried by slower water and laid on the bed, and over the land when the river floods.
• The river bed of deposits is the active floodplain; the land above the banks is the inactive floodplain, with flood deposits (fine silt and clay) and channel deposits (coarser, in abandoned channels).
• Floodplains in a delta are delta plains.

• Natural levees: low, linear ridges of coarse deposits along the banks of large rivers, often broken into mounds.
• Point bars (meander bars): sediments laid along the inner bank of meanders of large rivers, fairly uniform in profile, of mixed sizes.

Meanders. On flood and delta plains, large rivers rarely run straight; they form loop-like channels. A meander is a channel pattern, not a landform. Causes:

• water on a very gentle gradient works sideways on the banks;
• loose alluvium in the banks, with irregularities the water can push against;
• the Coriolis force deflecting the moving water.
• Deposition goes on along the convex (inner) bank, which slopes gently; undercutting along the concave (outer) bank, which becomes a steep scarp (the cut-off bank).
• As loops grow deeper, erosion at the inflection points can cut a loop off, leaving an oxbow lake.

Example (new): at a bend in a village river, children swim on the gentle sandy side of the curve, while the far side has a steep, crumbling bank where the current hits hardest: deposition on the inside, erosion on the outside.