Visitors to Mill Creek Canyon will have to pay $2 per carload beginning May 16 - a move Salt Lake County commissioners said is necessary to keep the canyon open.

The county and the U.S. Forest Service will collect fees until Oct. 31 and again on weekends and holidays in November and December. Only Boy Scouts and Girl Scouts, pedestrians, bicyclists and people who live in or have cabins in the canyon will be exempt from the fees. Seniors and disabled people will be charged $1. Entrance will be free on Tuesdays."Unless we do something, the canyon will have to be closed," said Commissioner Randy Horiuchi, who made the Mill Creek issue a campaign promise.

People have been using the canyon so much that it is beginning to erode, and neither the Forest Service nor the county has had the money to take care of it. The fees are expected to bring in $703,000 in 1991. Some of that will be used to build a toll booth and to pay someone to take the money.