Great question! The answer is both, and it depends on the geometry of the specific river - not to mention how it interconnects to branching/merging paths of other river segments to really complicate what's going on.

I've done a fair amount of white water kayaking, and we always pay attention to current rainfall conditions, and even if an upstream dam has been recently released. High water conditions can be very fun as some stretches flow faster and make certain rapids larger. But I've also seen high water conditions completely wash out a drop to feel like a speed bump.