The victims were returning home after visiting a shrine nearby when the accident happened near Ghamaur village in trans-Ganga region, they said.
Six died instantly and three in hospital, police said adding the dead were aged between 14 and 55 years.
The injured were in critical condition and hospitalised, police said.
The tractor-trolley was carrying some 24 people when it plunged into a gorge filled with water following heavy rain in the area in the past two days.