Residents of the Red River Valley waited nervously behind miles of spongy dikes shored up by four million sandbags yesterday, while a record deluge in Florida flushed poisonous snakes, washed through posh resorts and flooded homes and high-ways.

Miami had its largest rainfall in history, with the National Weather Service reporting a 24-hour total of 16.21 inches at 1 p.m. Fort Lauderdale reported 14.32 inches and Homestead 10.77 with the rain still falling. The rains ended a three-month Florida drought, the worst since 1971.

A plane with four people aboard was missing, an exclusive resort hotel suffered heavy fire damage at the storm's height and a helicopter mission rescued two crewmen from a grounded asphaltladen barge rocked by 22-foot seas.

Police in Plantation, west of Fort Lauderdale, warned parents to keep chldren indoors and out of the way of snakes routed from their hideaways. Two snakebites were reported, but it was not known if the snakes were poisonous.

On the brighter side, the Pearl River at Columbia, Miss., while still about nine feet above flood stage, continued to drop and some of the town's 4,000 residents who had fled were able to return home.

In the flat and sprawling Red River Valley shared by northern Minnesota and North Dakota, flood waters at record depths have spread as far as 1k miles wide and 25 miles long in some areas, covering 425,000 acres in North Dakota and 274,000 acres in Minnesota. Entire communities are isolated, including 3,000 persons marooned in East Grand Forks, Mini., at the confluence of the Red Lake River and the Red River.

The Red River was running at 48.94 feet, almost 21 feet above flood stage, and was not expected to crest until today or tomorrow. CAPTION: Picture, Waters rise around Hollywood, Fla., phone booth. UPI