Wiktionary
n. (context geography English) A plain formed by the deposition of sediment from the periodic flooding of a river.
Wikipedia
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 Glossary of Landform and Geologic Terms, maintained by the National Cooperative Soil Survey, defines an "alluvial plain" as "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".
Usage examples of "alluvial plain".
I leave the flat alluvial plain behind and climb into the badlands, angling toward Eisenbrucke Station at reduced speed.
They drove the horses hard, so that when they raced down the escarpment of the eastern hills on to the wide alluvial plain of the river, the lather had dried white on their heaving flanks and their eyes were red and wild.
The field was bordered by a rise, a terrace of the alluvial plain marking an old shoreline of the Hemmar in some age long before men came to Bellevue.
The iron foundry of one of the most famous gun manufacturers in all the Punjab and the Indus basin which meant in all India was located on this flat and featureless alluvial plain.
At this time of year I'd head towards the setting sun until I left the mountains and reached the alluvial plain where you'll see evidence of drumlins and some quite fine examples of obviously erratic boulders.
Beyond lies more open ground, where badlands have given way to alluvial plain, covered with tough grass which lies flattened, now, beneath a mantle of snow.
Six hundred miles farther east the river entered the territory of the Dravs, and ran through a wide alluvial plain which was inundated in the season of rain.
Now, a few million years ago when the Rockies were as high as today's Himalayas, the Ogallala was a vast alluvial plain created by silt runoff from the Rockies to the west.
The water was clear for a river running through an alluvial plain, very clear once he was away from the bank, and he swam downward until he felt the strong cold hand of the current gripping him.