An alluvial plain formed by fluvial sedimentation. Rivers carry large amounts of sand and silt from their upstream regions, and the flow velocity decreases in the lower course due to the gentle gradient. As a result, sediments accumulate along the river banks to gradually form an alluvial plain. In general, alluvial plains refer to the extensive plains that form by proluvial deposition and the expansion of deltas. They include piedmont plains, alluvial plains and coastal plains. An example is the North China Plain. In a narrower sense, alluvial plains only refer to those typical alluvial plains that form by long-term deposition along river banks triggered by shifting of the river courses during flood periods.