I would dump a truck load or 2 of dirt right there and make me a dam!
damming the finger by making a damn that is within your property line might be a good option as someone had suggested.
Be careful with this thought. Restricting the free flow of drainage water from another property owner is a no-no. We ran into the exact scenario while looking at property in central Illinois. After building the pond, one stinkin' draw would back up water onto a neighbor's property. We learned that this would give the neighbor access to the entire body of water. Fine; I would just push up dirt into the draw on my side of the property and seal it off with a mini-dam. Wrongo...can't restrict the natural drainage of the neighbor's property.
