Between 4.45pm and 7pm a red Land Rover Discovery was prevented from crossing the dam by a quad bike parked across the road.
As it waited, a large rock was thrown through the window of the vehicle and a woman tried to enter it via the window, but was prevented from doing so by the occupants.
This lasted for several minutes and generated a lot of noise. Threats were also made to push the Land Rover into the reservoir.
The Land Rover remained stuck until about 7pm.
While this was going on two motorcyclists were also prevented from crossing the dam. They remained nearby until just before 7pm when they rode away along the unsurfaced road towards Dead Man’s Hill.
Officers want to speak to the motorcyclists as they could be important witnesses to the incident.
If you are the motorcyclists, or anyone who has any information about the incident, please call North Yorkshire Police on 101, select option 2, and ask for PC Bill Hickson or Sergeant Andrew Tiffany.
You can also email firstname.lastname@example.org
Please quote the North Yorkshire Police reference number 12170180205.
Officers would like to provide reassurance that this is an isolated incident and the wider community were not at risk.