The word "rapid" here is not an adjective but a noun.
The noun "rapid" is PART OF A RIVER where the bed slopes down and the CURRENT is fast.
A gentle "rapid" is a part of the river where the water slopes down but it can be navigated by a small boat.
In English, there are rapids that are gentle and rapids that are strong and difficult for boats to manage.
There is actually an International Rating system for rapids.
Class A is "still rapids". Class 6 is "extraordinarily difficult rapids".
Hope this helps