what is the average humidity in your area?
the lower the humidity the better your evaporative scheme will work, however once the humidity gets up to a certain point the evap begins to decline rapidly to the point that there will be very little cooling.
my vote is +1 for a fast growing shrub or vine/trellis, make it something that produces something edible and you kill two birds with one stone.
once established, they mist the vine/bush with water and it should be very cool on the shady side in a dry climate.
as for the water diversion, i would simply take the water, use it and then recycle it back via a small pump. that way there is no concern regarding dumping it back into the creek.
bob g