Which river feature in the Northern Plains increases deposition?

Correct answer: Reduction of velocity after entering plains

River deposition occurs when a river loses enough energy to carry its load. In the Northern Plains, a river leaves the steep mountain valleys and enters a broad, gently sloping plain. Its speed generally decreases there, so it cannot carry as much sand, silt, gravel, and clay as before. The transported material is therefore dropped on the river bed, banks, or nearby floodplain.

Option A is correct because reduction of velocity directly encourages deposition. The other choices do not describe the main change that makes a river deposit its load: increasing sea salinity, a straighter mountain valley, or very hard rocks does not by itself explain the deposition in the plains. A river may also deposit material when its discharge falls or its channel becomes wider, but the given option identifies the essential relationship between lower speed and deposition.