An alluvial plain is a largely flat landform created by the deposition of sediment over a long period of time by one or more rivers coming from highland regions, from which alluvial soil forms. A floodplain is part of the process, being the smaller area over which the rivers flood at a particular period of time, whereas the alluvial plain is the larger area representing the region over which the floodplains have shifted over geological time.
As the highlands erode due to weathering and water flow, the sediment from the hills is transported to the lower plain. Various creeks will carry the water further to a river, lake, bay, or ocean. As the sediments are deposited during flood conditions in the floodplain of a creek, the elevation of the floodplain will be raised. As this reduces the channel floodwater capacity, the creek will, over time, seek new, lower paths, forming a meander (a curving sinuous path). The leftover higher locations, typically natural levees at the margins of the flood channel, will themselves be eroded by lateral stream erosion and from local rainfall and possibly wind transport if the climate is arid and does not support soil-holding grasses. These processes, over geologic time, will form the plain, a region with little relief (local changes in elevation), yet with a constant but small slope.
The above description seems to fit with the primary definition found in the "Glossary of Landform and Geologic Terms", maintained by the National Cooperative Soil Survey, which states that an "alluvial plain" is "a large assemblage of fluvial landforms (braided streams, terraces, etc.,) that form low gradient, regional ramps along the flanks of mountains and extend great distances from their sources (e.g., High Plains of North America)" Use of "alluvial plain" as a general, informal term for a broad flood plain or a low-gradient delta is explicitly discouraged. The NCSS glossary instead suggests "flood plain".
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Other articles related to "alluvial plain, plain, alluvial":
... In the alluvial plain, soil quality has been damaged by the deposit of large amounts of salts, borne by irrigation overflows and wind and promoted by poor soil drainage ... which drained the extensive marshes in the lower reaches of the alluvial plain, changing water circulation and wildlife patterns over a wide area ... Flooding danger in the alluvial plain has decreased since construction of dams upstream on the Euphrates ...
... Babylonia, Iraq Mississippi River alluvial plain Oxnard Plain, Ventura County, California Padan plain, Italy ... Punjab alluvial plain Laguna de Santa Rosa Christchurch, New Zealand List of alluvial sites in Switzerland Fertile Crescent ...
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