The downstream portion of a river . A. has lower discharges than do upstream portions B. features greater hydraulic action than do upstream portions C. generally becomes more sluggish D. generally is of higher velocity, although it is masked by reduced turbulence

The correct answer is option D. "generally is of higher velocity, although it is masked by reduced turbulence".

Explanation:

In most rivers, the downstream portion is both wider and deeper than the upstream portion. This means that there is more water in the downstream portion, which in consequence results in the river having a higher velocity. However, this is masked by a reduced turbulence since a higher amount of water tend to overcome more easily all the obstacles that can generate friction.