Introduction to Physical Geology Practice Exam

In open-channel flow, discharge Q equals velocity times cross-sectional area (Q = V × A). Narrowing the channel reduces the cross-sectional area A. If no water is added or removed along that stretch, the same amount of water must pass per second, so the velocity V must increase to keep Q the same. That’s why the correct idea is: velocity increases, discharge stays the same.

If water were entering the channel or leaving it along that reach, discharge could change, but narrowing alone changes area and hence forces the velocity to rise while keeping discharge constant.