i used the stuff which you use to plaster your tiles against the wall, i don't know how you say it in english. I think it is called mortar fixative. First i made a body out of styrofoam, and i totally coated it with mortar fixative, including the parts where the substrate is located, this because otherwise the substrate would go into the styrofoam.
These are two pictures
At this picture you see the first part that is totally coated, and on the left you'll see the pumphousing. Total ammount of water in the system is only 8 liters. The middle of the river is the point with the biggest waterdepth, so you can empty the river, and clean it.

And a close up of the waterfall
Further on i've made a housing for the pump a the left side (on the picture). I camouflaged the topping of the housing as a stone. This way i can easely clean the pump. At the end of the river, where the pumphouse starts, i've put a small filter in the water, this is , so the frogs can't swim into the pumphouse.