How to have no particles at certain places?

Hi!

This is amazing! Thank you for all the work on this plugin for mapbox. I am trying to visualise the movement of water in a river channel using this visualisation. So, i want to have the particles only in the river, so the land part of the map should not have any particles. 

If there is no velocity vector for the location, what should the color encoding be? 

Thank you! :) 