Learn Dutch: Het water in de rivier kabbelt rustig. - The water in the river ripples quietly.

The verb kabbelen means to ripple or babble, describing the continuous, small, repeating sounds and movements of water. The -elen suffix gives it this iterative (repeated minor action) meaning. Unlike verbs like druppelen (from druppen), the base root for kabbelen isn't really used on its own in modern Dutch anymore, but the suffix still does its job of expressing a continuous, gentle rhythm.

Can I use kabbelen to describe repeated small movements of things other than water?

Literally, it is almost exclusively used for water. However, at a B2 level, you will often encounter it used metaphorically to describe things that progress slowly, steadily, and without drama. For example, een kabbelend gesprek is a relaxed, softly flowing conversation, and de film kabbelt voort means the movie meanders along peacefully (which can sometimes imply it is a bit uneventful).

Why is the subject het water in de rivier grouped together before the verb, instead of saying Het water kabbelt rustig in de rivier?

Both are grammatically correct, but they mean slightly different things. In this sentence, in de rivier is part of the subject noun phrase; it specifies exactly which water we are talking about (the water in the river). If you wrote Het water kabbelt rustig in de rivier, the phrase in de rivier becomes an adverbial phrase telling you where the action happens, rather than identifying the water itself.