The Senate was droning through a laboriously long-winded debate on Title 18 of the U.S. Code one day last week when the majority leader, Sen. Robert C. Byrd (D-W.Va.), interrupted with a brief announcement that electrified friends and fose of S.790, the waterway toll bill.

Spurred by the heartfelt plea of a frustrated colleague, Byrd told the Senate that he would schedule a final vote sometime this week on the controversial bill, which would impose for the first time a user fee for commercial barges moving freight on the nation's inland waterways.

For the senators, staff aides and lobbyists who have been working 10 months in support of or in opposition to the waterway toll, Byrd's announcement came as a shock. They had expected the bill reach the floor early this year, but now they would have only days to marshal their arguments and push for votes.

Sen. Pete Domenici (R-N.M.), S-790's sponsor, and the railroad and environmental lobbies aligned with him, were pushing the Senate to approve once again the waterway bill it approved last June.

That measure established a tough fee schedule that would recover more than half of the hundreds of millions of dollars the federal government spends annually on inland waterways.

On the other side, Sen. Russell B. Long (D-La.) and a coterie of barge industry lobbyists and farm groups were asking senators to vote instead for a considerably different waterway tax that the House passed in October.

The House bill would be much less a burden on the barge operators, and would return less money to the government.

The barge industry isn't exactly enchanted with any waterway fee. But Domenici had trapped it.

Early last the year senator had attached his waterway proposal to an authorization bill funding a major new barge facility on the Mississippi at Alton, Ill. The barge operators had never managed to separate the two and now they had no choice: to get the Alton project, they would have to pay some level of waterway fee.

The barge lobbyists were taking a calculated risk in asking senators to vote for the modest House version of the waterway fee, because last fall President Carter jumped into the fray on Domenici's side.

In a letter signed by Transportation Secretary Brock Adams, the administration promised it would veto any authorization for the Alton barge facility unless Congress passed a waterway fee higher than that proposed in the House bill.

But Long and his supporters, including Sen. Adlai Stevenson (D-Ill.), maintain that the President is bluffing. "I think the Senate should approve the House-passed waterway fee," Stevenson said last week. "And I think the President, on reconsideration, would gladly sign it."

The waterway bill has been pending on the Senate floor since last October, but it had been in a legislative limbo because no one was pushing to bring it up. Neither Long nor Domenici was confident he had enough votes to win. Both men had been biding their time.

It was a neutral party, Sen. Mike Gravel (D-Alaska), who moved Byrd last week to schedule a vote.

As chairman of the Senate's water Resources subcommittee, Gravel has sat through hearings year after year on the issues of the waterway toll and the Alton barge facility.

"None of this has anything to do with Alaska, you know," Gravel has observed. "And the thing never goes away. It's a hair shirt; what have I done to deserve this."

When Gravel saw that both sides were stalling, he panicked. If the impasse went on much longer the whole waterway mess might be thrown back to him for more hearings. When he saw Byrd on the floor last week, Gravel made a personal plea for a quick vote that would decide the fate of the waterway fee.

Byrd, who was eager to clear away as many minor bills as possible before the Senate swims headlong into the Panama Canal debate agreed. The waterway toll bill was once again on its way.