I'm nearly finished my renovation of this display.
This section had been overrun with a plant I absolutely hate, Acorus Gramineus or Sweet Flag. This plant is generally grown in water or waterlogged soil and is extremely tough. It's also apparently edible and extremely nutritious, I don't care because it stinks and is horrible to remove.
Today a dug-out sections of the Acorus that had clogged up the flow of water, reprofiled the edges and topped with more stones.
I only have the small section under the waterfall and a couple of spots to the back of it left to do.