it could be that there is an easement which would allow for access to the water at any time you need to check as it wont stop you from extening your fence to your propertys boundries ive experianced this before but as long as you leave an access no harm no foul you can check at your countys building dept to see if a easement is in place it might explain your fence layout or the previous owner had something else in mind its worth your time to check if you fence out an easement the legalities can become nightmare-ish

My house was constructed about 8 years ago, and the fence isn't covering all of the property. i look at the deed that came with the house and it shows that more then half the back yard isn't being covered, and a bit of the front yard is being cut off as well. it seems like their is some government fencing around because we live in and around a creek. and i see were it was cut off was around a tree conservation area, will that be a problem? their is nothing behind my house but thick woods with a small creek. i want to know if it is under assumption that i am supposed to care for it or not. when we moved in their was a clear path right to the creek itself is that some sort of property line? i am planing to fix the fencing so i can try to manage the land better, how do i have to notify?