Convert the linear measurements from mm to ft: there are 304.8 mm in 1 foot, and 1 m = 1000 mm. Multiply the dimensions of the channel to get its cross-sectional area, and then multiply this by the linear flow rate to get the volume flow rate.
8 metres is 8000 mm
In 1 second 844 x 586 x 8000 cubic mm of water flow down the channel.
There are 305 mm in a foot (you might want a more accurate ratio than this) so 305 x 305 x 305 cubic mm in a cubic foot.
divide the water-flow by this number and you get the flow in cubic feet/sec.
So is the channel rectangular/cuboidic.
From night school memories I would do like Etch says but to save to save conversion until the end.... so find volume flow in mm^3/sec (844x586 x 8?) and then divide by whatever the conversion factor is to get to cubic feet/sec