A friend of mine spanned a 25ft wide, 8ft deep gorge with a bridge. he used a 40ft flatbed semi trailer minus the running gear. Your smaller creek could be spanned with a 14 or 16ft flatbed off a straight truck real easy. They are bull strong and if you get lucky and find an old one with steel flooring, you'll never have to repair it. I'm sure you can buy an old flatbed way cheaper than building a bridge out of lumber. My friend dug his into the ground so it's level with the trail grade. Very quaint, out in the middle of a huge timber.