I wouldn't create a dam. I would create something with concrete bags that I could walk across, drive a 4 wheeler across, etc. You will have to use quite a few of them to keep it from washing out though. I would anticipate that you will need them about 4 deep. Make sure you put them as flat as possible or water will run under them and defeat the project.
I don't really see a problem. As soon as you back water, it will run over the walkway.
