I had a superb suggestion off mrrforums so I put in some debris on the dam to explain the gaps in the waterfall. I used some grass stems and fluff from the centre of my tree fern – sadly not sprouting yet this year after the last year’s cold :(.
I used modpodge to put in some waves on both water levels:
I also put modpodge on the debris to glue it in place. I may trim that central clump back a bit: