Final answer:
The statement that sediment enters a river primarily through mass movement and weathering erosion is true, as these processes contribute to the transportation and deposition of sediments in river systems.
Step-by-step explanation:
The statement is true; sediments can enter a river mainly through two processes: mass movement and weathering erosion. Mass movement includes the downhill transfer of soil and rock material under the influence of gravity, and it happens rapidly in events like landslides or slowly as in soil creep.
Weathering erosion, on the other hand, is a more gradual process where rocks and soil are broken down by elements like water, wind, or ice, and then transported by the means of erosion, such as river water. Once material is eroded, the river carries these sediments downstream, depositing them in areas where the river's flow diminishes, creating various landforms such as floodplains and deltas.