The experts will surely find a less cumbersome solution to this, but this works.
First, set the node on the "Anchor" / "Sit on terrain" tab of the "Pop Grass01_2.tgo" to "Redirect Shader".
Not the "Compute Terrain".
That feeds the shape of the river into the population.
If you use the same node in the "Use density shader" it already populates the plants to the edge of the riverbed.
But somehow one has to raise the "Object spacing in a,b" quite high to fill the space.
Because of that I tried to get a mask out of the "Redirect shader".
I won`t describe how, take a look at the project.
Btw, I changed a lot of settings in your file to get faster response and renders, so be sure to use your own tgd again and rebuild what you need.
Hope that helps.
CHeers, Klaus