One man was lost and believed drowned and 450 homes and thousands of acres of farmland were under 3 feet of water today after an irrigation dam burst on Thursday in the flood-swollen Sevier River 125 miles southwest of here.

The dam break near the central Utah town of Delta is the latest in a series of flood-related incidents that have plagued Utah this spring. Already more than half of the counties in Utah have been declared federal disaster areas because of the flooding.

More than 15 months of record-breaking precipitation have triggered mud and landslides, temporarily turned major streets in Salt Lake City into "rivers" and raised the level of the Great Salt Lake to its highest point since 1922.

"It's a hell of a way to run a desert," said Gov. Scott M. Matheson in a recent speech.

The legislature convened in a special session this week to look at ways of paying for the estimated $200 million in damage that had already occurred before the Sevier River dam gave way.

The 30-foot-high earthen dam that broke on Thursday is called the D.M.A.D. Dam, for the Deseret, Melville, Abraham and Delta Irrigation companies which operate it.

Record flows in the Sevier last week had washed away a small irrigation dam below the D.M.A.D., causing a deep channel to begin eroding back toward the larger dam. When the channel reached the D.M.A.D., it quickly undermined the concrete spillway.

"The cement structure just tipped over like you were opening a garage door. It went in about 30 seconds," said Vince Cropper, a rancher who watched the dam collapse.

The flood waters skirted the edge of Delta, but early today they spread through the low-lying farming communities of Deseret and Oasis, inundating fields and homes with as much as 4 feet of water.

An unidentified man was believed to have drowned today in an attempt to pull himself across the river on a downed telephone line.

Damage to homes in the area was moderate, but farmers were concerned about recently planted crops and destruction of their elaborate irrigation system.

"Well, we're getting irrigated now," joked Evan Skeen, a Deseret farmer whose hayfield was converted to a shallow lake by the flooding.

"I guess it's all right. It won't put me out of business, but it will hurt. My livestock are probably all dead and my house is under water," he said.

The National Guard was called to help with the cleanup and in rebuilding some of the dams. State officials are anxiously watching the larger Yuba Lake Dam upstream on the Sevier River, where runoff has filled that reservoir to record levels.