What is the main reason for greater deposition in the lower course of a river?

Correct answer: Decrease in gradient and carrying capacity

Direct answer: Option C is correct. Deposition is greatest in the lower course mainly because the river’s gradient and carrying capacity decrease. In the upper course, the slope is usually steep, so water flows rapidly and has strong energy. Fast water can carry larger and more material. As the river moves toward the lower course, the slope becomes gentler, friction and a wider channel reduce speed, and the river loses energy. When the river can no longer carry all its sediment, it drops the heavier particles first and deposits more material. Option C correctly connects lower gradient and lower carrying capacity with deposition. Option A is wrong because ordinary lower-course deposition is not caused by water freezing. Option B is wrong because very high gradient and velocity generally increase erosion and transport rather than causing the main lower-course deposition. Option D is wrong because removal of sea water is not the controlling reason for river deposition. The exact amount of deposition can also depend on sediment supply, floods, channel shape, and the river mouth, but among these choices C is clearly the best answer. Memory cue: less slope, less speed, less carrying power, more deposition.