You do need an easment at a minimum.
As a point of fact, even if you do have an easment your neighbor can also do somethings if his property starts to see a back up of water. He can re-route it so that your backed up portion is no longer part of the creek, He can, as was stated above, float anything he wants down the creek and into your pond as a navigable water way. He can introduce any fish or critters into your pond he may want to by introducing them into "His Creek". There could be accidental fertilizer spills that make it into "His Creek" That cause major blooms in your pond.
Or you could run into what I ran into which is a neighbor who claimed the water backing up 2 inches higher than it normally did in the past caused major issues with septic and other things. My neighbor( as some of you already know ) decided to take measures into his own hand and destroyed my dam on my property.
Thus even with easments you are running a major risk of having neighbor issues and problems that can get to the point of lawsuits, shotguns, lynchings and maybe more!!!!
Thus as for one who has gone through it. AVOID THIS AT ALL COSTS !!!!!!
