What defines an alluvial plain?

An alluvial plain is defined as an area of fertile soil that is deposited by the sediments carried by river floodwaters. This sediment comes from the erosion of surrounding landscapes as rivers flow, often containing a mixture of sand, silt, and clay. When rivers flood, they spread their waters over the plain, depositing these nutrient-rich materials, which can create highly fertile land suitable for agriculture. This process contributes to the rich biodiversity and productivity often found in such regions.

The other options describe different geological and environmental phenomena: merging tectonic plates relate to mountain building and seismic activity, dry landscapes with minimal vegetation refer to arid regions or deserts, and volcanic activity creates landforms like mountains or lava plains. None of these options align with the characteristics of an alluvial plain, which is primarily defined by its association with water-induced sediment deposition and its resultant fertile soil.