Final answer:
The neighbors could claim an easement by prescription to require the owner to remove the fence.
Step-by-step explanation:
The easement that the neighbors could claim to require the owner to remove the fence is an Easement by Prescription.
Easement by prescription is a type of easement that is acquired through the open, continuous, and hostile use of another person's property for a certain period of time. This means that if the neighbors have been using the footpath to access the river for a long time without any objection from the owner, they may have acquired an easement by prescription.
The neighbors would need to prove that they have used the footpath openly, continuously, and without permission from the owner for a specified period of time, which varies depending on the jurisdiction. If they can successfully establish these elements, they can legally claim an easement by prescription and request the removal of the fence, as their right to use the footpath has been established through their long-standing use of it.