What term describes fertile sand-sized particles found in the lower course of a river?

The term that describes fertile sand-sized particles found in the lower course of a river is silt. Silt particles are typically smaller than sand particles but larger than clay particles, measuring between 0.002 mm and 0.05 mm in diameter. These fine particles often come from the weathering of rocks and organic matter, and they can easily be carried by water, making them abundant in river systems, particularly in the lower course where the flow slows down and sediment settles out.

In the context of river environments, silt is crucial for soil fertility. When sediment is deposited, silt contributes to the nutrient-rich topsoil that is ideal for agriculture. This is why floodplains and deltas, which are areas with high silt deposits, are often some of the most agriculturally productive regions.

The other options do not accurately describe the specific size and characteristics of the particles in question. Clay is too fine and would not be classified as sand-sized. Sand particles, while they are larger than silt, do not possess the same nutrient-rich qualities associated with fertility. Gravel, being even larger in size, is also not considered to be fertile in the same way as silt and would not refer to sand-sized particles.