Stan:
Once again all I can say is "Ditto" to Dicks post....Having said that, I'm like you, I like the "au natural" look of a waterfall and streams, have um on our Japanese Garden pond...
When I first installed it, I lined the stream beds with 2" flat river rocks, looked great, but after only a week they collected too much fine crud...took um out and went with just the concrete beds, now clean as a whistle, no creepy crawlies...algae makes them look natural...
A seasoned hobbyst posted a thread on the NI board awhile back explaining that his water quality looked great, but then the koi started doing the back stroke...problem, debris on the bottom of his catchment basins which hadn't been cleaned in some time started to ferment adding unseen toxins to water...
Aloha! Mike