“There is an easement across our property that gives access to two other properties. Recently there has been a change of ownership on one of the properties. The new owner has a teenage son. The parents leave town often and leave the son alone. While the parents are away the son has been throwing parties where there is teenage drinking. The parties have been busted up several times by law enforcement and the parents are aware that this has been occurring but still they continue. Part of the easement across our property also crosses water. We are concerned that at some point one of these underage drunk children will cause and/or have an accident that will result in death or injury while crossing our property. Can we be held liable if something like this should occur? Other than calling law enforcement to alert them to the party is there any thing else we can do to protect ourselves?”
Yes, there is a risk of liability if someone is injured on your land, even within an easement. The degree of this risk depends on what state you are in, and the specific facts of the accident.
You could try to make a legal argument that the scope of the easement was not meant to include large numbers of guests. Depending on precisely how the easement was written, you might be able to get somewhere with that argument, with result being that these guests are essentially trespassing, and you could call the police and/or sue to prevent the trespass.
Honestly, I don’t think that’s a great argument. Residences are expected to have guests at times, so allowing guest use of the easement is probably implied.
Perhaps a better argument would be to point out to your neighbors that if there is an incident, your insurance company will doubtless draw them in as possibly liable, as well. This might motivate them to take a stronger position on teenage parties.