How would you find the discharge of this example

I'm trying to understand how to find discharge.
Let's say there's a dam forming an upstream reservoir of Yahoo River - a 5th order stream (just an example). If the drainage basin is 2000 mi^2 and the velocity of the river is 4ft/s where the river is 6 feet deep and 200 feet wide, what is the discharge?
Additional Info:
avg rainfall is 50 in/yr
Reservoir is 206 feet deep
Runoff is 40% of accumulation. The other 60% will soak into the ground before reaching the dam.

Please Help!
Thank You :)

There's a lot of data in your question that are not required to answer the question, "What is the discharge?" provided you are measuring the discharge at the point where we know the velocity, width, and depth. That is all we need to calculate the flow at that location.

Area = Width x depth
A = 200 x 6
A = 1200 sq ft.

Velocity at that location is 4 ft/sec, so just multiply 4 x 1200, and we have flow (Q) = 4800 cubic feet per second.