Can I expect to have bigger or smaller rapids in a river that has a water level that is higher than average? I am not sure which way this would go. With the lower water, one is closer to the rocks, which, I think, generally are the things that create the rapids. But perhaps a higher water level is associated with a faster current, which would seem to make rapids greater.
Most (natural) rivers contain different sizes of rocks. The smaller rocks will be closer to the ground and the larger rocks will by definition also be taller. So different water levels will allow different sizes of rocks to form different kinds of rapids.
For example at lower levels smaller rocks will form smaller waves and holes and the river will flow around large rocks.
As levels rise, the effects of those small rocks will become smaller and at some point the water will no longer flow around the larger rocks, but start to flow over them, forming large holes.
So if your river only contains small rocks, rising levels will increase the velocity on the water surface but will make the river smoother.
If the river contains small and large rocks, they will have completely different effects at different water levels and might form new river features as the water level rises.