My open channel hydraulics are a little rusty having ended up in project management for a few years.
Assuming sub-critical flow, i.e. the Froude number is less than 1. This is reasonable unless the inlet to your channel is horizontal and the water is entering at high velocity.
The depth of flow is governed by a control structure at the down stream end. Two conditions could exist - either the water free falls into the basin, or the discharge is flooded and the downstream level is the same as the basin.
Assuming free-fall, calculate the critical depth of the flow. This is the same as a weir equation, setting the height of the weir above the invert of the channel as zero.
The normal equations for flow in an open channel fall apart when the depth is critical. To overcome this, go say 3 depths back upstream and add 10% to the depth. Now split the remaining length of channel into small segments, and working upstream calculate the frictional losses in the next segment, hence the change in total head, hence the change in depth. Use the new depth for the start of your next segment. Do this all the way upstream to the inlet.