Alluvium refers specifically to the sediments that are deposited by flowing water, such as rivers and streams, after the process of erosion. Over time, as water flows, it carries particles and sediments from upstream sources. When the velocity of the water decreases—such as when it enters a floodplain or an area with reduced gradient—these sediments settle out, forming alluvium.
This material is typically composed of a mix of clay, silt, sand, and gravel and plays a significant role in the geologic and ecological features of river basins and floodplains. Its accumulation is crucial for creating fertile land, particularly in areas where rivers meander and flood.
The other options describe different processes or phenomena. Gradual withdrawal of water is related to hydrology but does not define alluvium. Land formation from gathered soil might refer to other processes like soil formation or sedimentation, but it doesn’t specifically identify alluvium. Change in river course might result in alluvial deposition but does not accurately define what alluvium is. Thus, the definition of alluvium as material deposited by water after erosion is the most precise and accurate understanding of the term.