It appears that you can measure the area of the channel, now you need to mesure its velocity of flow. By multiplying area and velocity together you will have the volumetric flow rate.

Note that the velocity measured at the top of the channel will not be be the same as the velocity at the sides and bottom of the channel due to frictional effects. As a broad estimate, assume that the average channel flow velocity is about 90% of that measured at the top.

I hope this helps.

Steve

I can measure the width of the tail race, and the depth of water flowing through it.. Any information on the applicable formula would be greatly appreciated.

Tom