There are lots of ways to simulate water, and they're all good. I used high gloss clear varnish over prepainted baseboard. Others will use different materials.
The type of stream you want determines the way you paint its bed. Mine is a clear trout stream. Ripples were worked into it as it dried.
I suggest you experiment on scrap board to see what effects you can get.
I use epoxy resin which is fine for sea and lakes but it would be tricky to use for a waterfall as it's runny. You would have to wait until it was almost off then trickle it onto the prepared waterfall area.
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