The incident took place on Friday when the forest guard tired to stop the mining mafias from lifting sand from the Kosi riverbed, a police official said.
The accused has been identified as Jasbir Singh and he was reportedly driving the dumper truck, Nainital SSP Janmejay Khanduri said.
Chief Minister Trivendra Singh Rawat had yesterday announced an ex gratia of Rs 1 lakh and a job to the wife of the deceased forest guard. He also ordered that the divisional forest officer and forest ranger be attached to the head office.
Demanding strict action against the accused, the Congress said a delegation of party leaders, led by Upadhyay, will visit the site of the incident tomorrow and also meet the deceased's wife.