Mask Objects

Currently, our bushes are spread across the entire terrain, which is not what we want. Instead, we are going to restrict them to the sides of the river, where water naturally collects on a dry terrain like this. To do this drag the river layer into the distribution of our object layer. This will create a River Distribution automatically linked to that layer. Also set the River Range to 0 and the Valley Range to 1 as we don't want our bushes to appear in the river itself.

As you can see, this gives us a very narrow mask around the river. To change this we'll set the Valley Width of the river to 5 so that we have a larger valley where bushes can be placed.

On top of that we now add a Blur effect to smooth it out even more and give our bushes a nice wide area to be scattered in.