Alluvial plain

a level or gently sloping surface formed of sediments laid down by streams, generally during flooding.
Historical Examples

Perhaps it is the nature of that alluvial plain that accounts also for so plentiful a harvest of mosquitoes.
The Fortunate Isles Mary Stuart Boyd

The stones at Beled had been the first signs that we were off the alluvial plain.
The Leicestershires beyond Baghdad Edward John Thompson

The alluvial plain of Babylonia was the gift of the two great rivers.
A Primer of Assyriology Archibald Henry Sayce

In this distance no rock strata appeared, nor any formation other than a jutting ridge of sand, or an alluvial plain.
Summary Narrative of an Exploratory Expedition to the Sources of the Mississippi River, in 1820 Henry Rowe Schoolcraft

By a slight ascent westward of the alluvial plain we reach Kilmartin, a village with a large modern church.
The Book-Hunter John Hill Burton

These terraces vary in height at different parts of the valley from 100 to 300 ft. above the alluvial plain.
Encyclopaedia Britannica, 11th Edition, Volume 15, Slice 6 Various

To the south is the rich, alluvial plain made by the delta of the Jordan.
Biblical Geography and History Charles Foster Kent

The alluvial plain extending behind the marshes was as rich and fertile as that of Chalda.
History Of Egypt, Chalda, Syria, Babylonia, and Assyria, Volume 4 (of 12) G. Maspero

The whole district consists of an alluvial plain formed by the deposits of the two great rivers.
Encyclopaedia Britannica, 11th Edition, Volume 10, Slice 2 Various

I have seen only one stream muddied in flood, the Choma, flowing through an alluvial plain in Lopéré.
The Last Journals of David Livingstone, in Central Africa, from 1865 to His Death, Volume I (of 2), 1866-1868 David Livingstone