ENNIS (AP) -- Engineers Tuesday tried to determine how best to control the flow of water through failed headgates at Hebgen Dam, a failure that allowed heightened flows and led to evacuation of some Madison River campsites as a precaution during the long Labor Day weekend.

Controlling the flow is necessary before the headgates can be repaired, said a spokesman for PPL Montana, owner and operator of the southwestern Montana dam.

Hydraulically controlled headgates at the 93-year-old dam failed on Sunday and allowed a fourfold increase in flows of water from the dam. Flows rose to levels normally seen during spring runoff. In addition to the evacuation of campsites, recreation on part of the Madison River was suspended.

The dam itself is sound, said Mark Lambrecht, regulatory affairs manager for PPL. Reports about the rushing water had led to concern that the dam had failed and imperiled the town of Ennis, about 45 miles away.

At The Blue Heron gift store in Ennis, sales associate Mary Van Pelt said she had moved books in case of flooding.

If a catastrophic dam failure ever occurred, Lambrecht said, Ennis would have eight hours notice before water could reach the town.

Campsites remained under evacuation orders Tuesday but fishing on part of the Madison had reopened, Lambrecht said.

The Madison River, normally flowing around 850 cubic feet per second at this time of year, flowed at about 3,400 cfs on Tuesday, he said.

The first report of trouble on Sunday came from a fisherman, said Madison County Undersheriff Roger Thompson. The fisherman reported that he heard a loud pop and a peculiar sucking sound, and saw a lot of water frothing at the headgates. Thompson said he and a PPL manager drove to the dam, where they observed that a 175-pound manhole cover had popped loose and was 6 feet from where it belonged.

As people who live along the river were told during the Labor Day weekend that they might want to prepare for possible evacuation, residents became alarmed. ''People started hearing rumors the dam was breaking,'' Thompson said. In Ennis, volunteers went door to door in Ennis to quell rumors.

Hebgen Dam underwent its annual federal inspection last Thursday and conditions appeared normal, said Carrie Harris, manager of hydro-engineering for PPL Montana.