I'm not sure if any of these suggestions are what you are after, so forgive me if they are not
:
the "potting soil" should stay in the containers and be saturated, so no watering.
Place rocks on top of the liner to hold it and to give the natural stream bed look.
If you drilled small holes in the containers and lined them with landscaping fabric before planting,
the "potting soil" should stay in the containers and be saturated, so no watering.
If you put the plastic liner down first it would hold the area from erosion.
Place rocks on top of the liner to hold it and to give the natural stream bed look.