Rescue teams were searching yesterday for at least five campers believed to have drowned when a reservoir burst in Rocky Mountain National Park Thursday and flooded Estes Park, Colo., the park's main entrance 11 miles away.

Three of the campers are thought to have died in the first avalanche of water that burst through a 100-yard gap in the 510-foot-high dam. A fourth was swept away in his sleeping bag.

The safety of the privately owned irrigation dam is the responsibility of the state, whose engineers last inspected it in 1978 and reported it in "acceptable" condition.

The engineers pointed out then, however, that the water level was unusually low at the time, and recommended that the next inspection be carried out when the levels were higher and the strain on the 79-year-old structure was greater.

Hal Simpson, the assistant state engineer, said yesterday that the inspectors have not found the cause of the rupture, which sent a wall of water 30 feet deep flooding through a campsite occupied by 200.

About 50 trailers upstream from Estes Park were badly damaged, and some shops in the town were swamped with up to five feet of mud. The steel-girded Olympus Dam on the southern side of the resort held the remaining water.

"It will be several weeks before we find out what caused the accident," Simpson said, adding that his department was responsible for the inspection of about 22,000 dams. "This is a very arid part of the country."

Liability for the damage, he said, lay with the owners, Farmer's Dutch and Reservoir Co. of Loveland, Colo.

Dam breaks in the country's thousands of reservoirs are not uncommon, and have been the subjects of government attention for the last decade. Congress first became concerned in 1972 after a flood at Buffalo Creek, W.Va., killed 125.

Small breaks occurred in a large number of other dams after Hurricane Agnes in June, 1972. Congress then passed the National Dam Inspection Act, which authorized the Army Corps of Engineers to make an inventory of the country's dams. This was done over the next three years, and 67,000 privately owned, state and federal dams were inspected.

In 1977, $15 million was set aside by Congress for another inspection, but the money was not appropriated. Later that year 36 people were killed when the Toccoa Dam burst in Georgia, and President Carter ordered the inspection program implemented immediately. The investigation was carried out over the next three years.

In April, 1981, the Corps reported that 8,794 of the dams inspected were classified as "high hazard," meaning that if an accident occurred the loss of life would be high because the dams were near urban areas.

Of these, more than a third were rated unsafe.

The Lawn Lake reservoir that burst Thursday was never inspected by the Corps because it fell com- pletely under state jurisdiction.