Stream as filter

Question
I have recently installed a 20' x 30' x 2' deep pond using EPDM.  I have a pump on the bottom which moves water to top of a 35' stream full of .375" - 1.5" river rock.  I have no intention of keeping fish in it, but my two ducks live on it and I have put in several floating aquatic plants.

1. Can the stream and the huge surface area resulting from all the gravel act as a filter or do I need a separate filter box?  There is around 6000 gal coming down the stream per hour so there is plenty of turbulence and aeration.

2. If the stream is my filter, would I be doing it any harm by turning off the flow for half the day?

Thanks,
Chris

Answer
Hi Chris,
Thanx for your question.  I should like to think that continuously moving water will be a lot cleaner than water that just sits.  Yes, I would go ahead with your idea.  I don't think there is any harm in turning off the pump for half the day.  I hope this helps.
tom