The channel is 2 meter deep and 4 meter wide. For every 1200 meters in channel length the elevation change is 2 meters. What is the flow rat

Does this help:

I've mistakenly used 200m for L instead of 1200m above.  With 1200m the flowrate becomes F ≈ 3050 kg/s.

Thanks Alan. :)

These physics ones always fascinate me.   

This is more engineering than physics, and I'm not sure I've made the simplifying assumptions the questioner was looking for!  Perhaps he/she will tell us.

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Yes it is always good when we get feed back of the question asker.