Fence Post Mystery

I was walking along a large slough and discovered a lone fence post.  It was an odd find as there didn’t appear to be anything to contain within a fence and the post wasn’t adjacent to a pasture where you would want to keep cattle out of the slough.  So why would you put fence around a slough?  Unless the slough didn’t exist when the fence was built.  The post’s bleached and somewhat decayed appearance suggests it’s been there a long time and the slough didn’t appear to be very deep.  Mystery solved?