The tank is to the right of the waterfall mound and there is a retaining wall between the mound & tank resulting in a gap between the two. If the tank was embedded in the waterfall mound a cut-out would be simpler but given the current design I think modifying the intake pipe will be easier. If I built a spillway I would need to create something to support the liner and route the water over the gap.Wouldn't it be simpler to make a cut-out in the plastic container?
I used this layout to maximize the size of the pond + waterfall given I have a relatively small urban backyard. I may need to revisit in the future.