* Erosion and Transportation: Rivers erode and transport sediments as they flow. The upper course of a river is characterized by fast-flowing water and steep gradients, which carry larger, heavier sediments like rocks and gravel.

* Sediment Sorting: As the river flows into its lower course, the water slows down and the gradient decreases. This causes heavier sediments to settle out first. The finer particles like clay and silt remain suspended in the water and are transported further downstream.

* Deposition: The lower course of a river is typically flat and wide, leading to the deposition of clay and silt in the form of floodplains and deltas.

In summary: Clay and silt, being the finest sediments, are most likely to be found in the lower course of a river due to the process of sediment sorting and deposition.