Have you tried Earth Learning Ideas 'Mighty river in a small gutter' which investigates how flowing water erodes soft sediment, or 'Rock, rattle and roll' which demonstrates that some rocks are more resistant to erosion than others?
Pupils at St Paul Junior School, Luyanzi, Uganda were lucky enough to study river erosion closely by visiting the Kalagala Falls on the River Nile. Here there are spectacular waterfalls and rapids in the river. The following photo shows the river being constrained by resistant rocks on either side.
Here you can see the start of the waterfalls:-
If you want to run some fieldwork outside, do try 'Earth science out of doors'.
If you have some photos of pupils doing fieldwork, we shall be pleased to display them.