Sediment is deposited as a river enters a lake because

(A) The velocity of the river decreases
(B) Force of gravity decreases
(C) The volume of water increases
(D) The slope of the river increases

The answer is number one. When the velocity (the speed) of the river decreases, be it at the entry of a lake, sea, ocean, or meandering, passing through a valley with very little change in altitude, the sediment starts to get deposited. This is due to the decrease in force caused by the decrease in velocity, there is not enough force to push the sediments further so they start to deposit because the river is not able to drag them further on.

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