From what I can tell, your watershed is huge, off the map you posted back to where the creek is sourced. Basically you map out the topomaps to the same elevation and above as the surface of your water will be. Imagine a line bisecting all of the ridges and hill tops surrounding the valley that makes up that creek on the topo-map. Anything that can drain into that creek.
You may have a different sort of problem, dealing with too much water during high-rain events. Design wisely my friend!
