President Reagan detoured to this flood-ravaged area today en route home from his vacation in Palm Springs, Calif., symbolically filled a sandbag and promised that he will declare stricken parts of Louisiana a disaster area, making the state eligible for federal emergency assistance.

Speaking on a regional radio hookup from a station surrounded by water held back by sandbags, Reagan urged voluntary contributions to the Red Cross and the Salvation Army and promised that "the federal government will provide every bit of assistance possible and without delay.

"We must do everything we can to help our neighbors in Louisiana rebuild their lives," he said. "I know the crest of the flood may not yet have arrived and that the situation may become still more difficult, but please know the rest of America is standing with you."

Louisiana Gov. David C. Treen estimated today that flooding has caused about $103 million in damage in six parishes (counties), including Ouachita, site of today's visit.

Deputy chief of staff Michael K. Deaver said Reagan decided Saturday to make the stopover to show his support for victims of floods that have ravaged Louisiana, Arkansas and Mississippi in recent weeks.

The White House continues to be sensitive about the "fairness" issue--as Reagan aides refer to the accusation that administration policies unfairly benefit the well-to-do--and especially about news accounts relating how the Reagans have vacationed in sunny climes during periods of high unemployment and economic distress.

Several such reports surfaced in the last week, which the Reagans spent in Palm Springs at the plush estate of Walter Annenberg. Since his inauguration slightly less than two years ago, Reagan has spent 175 days away from the White House, including 125 days devoted mostly to vacation.

On Saturday night, the Reagans enjoyed their final party of the week at the Vintage Club, a fancy Palm Springs country club. The president left in midmorning for Monroe and spent slightly more than an hour here before departing for Andrews Air Force Base.

Near a levee outside Mel's Diner here, Reagan lifted 11 shovelfuls of sand into one bag and four into another in a made-for-television ceremony that lasted 11 minutes.

He then walked to a Salvation Army camp and rode in a jeep through about two feet of water to the radio station for an address that interrupted the broadcast of the Atlanta Falcons-New Orleans Saints football game.

"That's all right," quipped Treen, a longtime Reagan ally who accompanied the president. "They'll probably lose anyway." Treen was wrong: the Saints won, 35-6.

When Reagan made a similar visit to a flood-damaged area in Fort Wayne, Ind., last March, he stressed the importance of voluntarism.

Although the president returned to this theme briefly today, his emphasis was more on the traditional federal role of providing economic assistance in flood emergencies. Reagan said Treen is framing a request that would declare portions of Louisiana a disaster area and said he will approve it.

"You have our respect, our prayers and a helping hand," Reagan said in the radio message. "With your courage and the nation's support, Louisiana will come through the flood, and then you'll do what is in every American's blood: rebuild with hope and determination."

In mid-December, Reagan declared portions of Arkansas, Illinois and Missouri disaster areas because of flooding.

The section of Monroe that Reagan visited today is not one of the area's hardest-hit neighborhoods. It was chosen because of its proximity to the airport, according to members of the Reagan advance team.

But, Treen estimated, damage in Ouachita Parish totals about $81 million, and this section of northeastern Louisiana just below the Arkansas border has suffered heavily from flooding. More than 9,000 people have been evacuated from their homes statewide.

The rain slackened today, and the National Weather Service predicted clear skies Monday.

The major source of flooding in Monroe has been the Ouachita River, now more than six feet above flood stage. Officials have forecast a crest two feet higher by midweek.