How can I scatter on some objects but not others?

What’s up everyone, I’m hoping someone can help me with the area populate tool. Using area populate, I drew the outline of the area I want to scatter some river rocks in. However, since it’s set to “snap to surface”, the rocks are populating on the surfaces above the landscape (as seen in the image). Is there a way to prevent the scattering from happening on the surfaces I don’t want them to be scattered on?