Final answer:
A decrease in sea level is unlikely to cause deposition of sediment along a river; instead, it may lead to regression and erosion if the sea level drops significantly.
Step-by-step explanation:
The question asks which of the following is unlikely to cause deposition of sediment along a river: 1) A decrease in gradient, 2) A decrease in water velocity, 3) A decrease in sea level. Factors that usually lead to sediment deposition in rivers include a decrease in gradient and a decrease in water velocity, as both of these conditions lead to slower water flow which allows sediments to settle. However, a decrease in sea level generally leads to regression, which would move sediment away from the coastline rather than causing deposition; if sea level drops enough to expose sediment, erosion will take place, opposite of deposition.