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

Can you give us a whole sentence? You are correct, this description is self-contradictory, therefore I guess it is a poetic use...?

The writer could be referring to "rapids" which are are fast and rocky/bumpy rivers. He may be using the word "gentle" to compare, basically, the rapids are usually fast but today they are gentle.