I think I understand now. I thought you were build a new wall to match the old. So you're just putting the waterfall up against the existing wall.
Yes, concrete block is the way to form the basic falls shape, then EPDM liner, then rocks.
If you really want this to last as long as possible the correct way is to dig down below the frost line. Your local building department can tell you how deep. Not sure where you live but it looks like a cold area. When you dig down you want to flatten the soil but only by removing soil. You don't want to put soil back in and tamp id down. Then you pour a concrete footer with rebar. There's instruction on the web if you decide to go that way, search "concrete footer". Then the block goes on top of that with vertical rebar tying the block to the footer. You fill the blocks with concrete. Horizontal rebar maybe mid course, use "bond beam" blocks in that course. Personally I just use bond beam for all the blocks, just easier when buying. A waterfall doesn't take many blocks.
You can fill in with rocks, stone, broken concrete, gravel. Dirt isn't good because it expands when it freezes. Plus it settles. Commonly waterfalls leak and often because of settling. The extra digging and material cost really isn't a lot because it's a really small structure.
Make sure you plan ahead for where the pipe to the falls will go.
Everything can be scaled back, but risk go up and lifespan is likely reduced. Most people, even professional pond builders which is a little shocking, never do anything close to a proper foundation. Mounding soil is very fast, easy and common and will generally last long enough (year or two) that the owner won't go after the builder.
No way to predict how long mounded soil would last, but I'd expect problems pretty quick.But they can be tweaked. Settling is the main enemy. Erosion can be spotted and fixed normally.
Interestingly when waterfalls are built in the swimming pool trade they virtually always are built will a proper foundation. They're in a trade that gets inspected a lot so they're used to building correctly. In the pond trade it only has to last long enough for the check to clear.
If you decide to go that way you can draw plans and post for tips.