The Senate began debate yesterday on the waterway bill, and for the senators it was an important occasion. This was pork barrel day. All day long the members come trooping into the chamber to get their share.

John C. Culver (D-Iowa) offered an amendment authorizing $224,000 for a bikeway along the Mississippi at Clinton, Iowa. The amendment was approved by "unanimous consent," a shortcut procedure that eliminates the need for debate or a vote.

J. Bennett Johnston (D-La.) asked for $2.2 million for a flood wall near New Orleans; approved by unanimous consent. Paul S. Sarbanes (D-Md.) wanted $379,000 to deepen the Choptank River; approved. Warren G. Magnuson (D-Wash.) wanted $18 million to dig a channel and raise a bridge at Tacoma Harbor; approved.

It was not unusual in the enale, where mutual back-scratching is an honored tradition, members rarely raise questions about one another's additions to the annual bill authorizing waterway construction projects.

But this year Democratic requests had particularly clear sailing, because the Republican who was on the floor to challenge them, Sen. Pete Domen ici (N.M.) was actually pleased with each new piece of "pork." It was all part of the strategy.

Domenici is the sponsor of controversial legislation that would establish, for the first time, a user charge for barge lines using inland waterways built and maintained by the federal government. The user charge legislation, S. 790, has been attached to the overall water projects bill, and that is the heart of his plan.

Although his proposal has been opposed by some of the Senate's most senior Democrats, the New Mexico Republican has garnered an important Democratic ally in his fight to pass the waterway fee: Jimmy Carter.

At a press conference Monday, Domenici announced that the president had threatened to veto the overall water projects bill if it did not include a waterway user charge close to the one Demenici had been seeking.

This Domenici sat in the Senate chamber yesterday smiling like a schoolboy who got five gold stars on his spelling test. Each new pork barrel amendment meant one more senator who would want to see the water bill signed into law. To make sure that happened, the senators would have to vote for Domenici's waterway fee.

"The more goodies we get in this bill, the mor people who will be afraid of the veto," the amiable New Mexican said late yesterday. "The way it looks now, if the president sticks tough on the veto threat, we're going to win."

It looked that way as well to a despondent group of barged industry lobbyists and river-state senators who have been battling Domenici on the sisue for more than a year.

As the barge lobbyists saw it, they had been sabotaged at every turn in the long fight over the user charge.

"Last spring, when Domenici first came up with this, we were against anything at all," recalled John Connolly, one of the barge industry's legislative strategists. "Then we said 'okay, we'll agree to something.' Then Domenici wanted more. Now he wants even more, and the president says the Senate has to go along. It's just not reasonable."

But there was still time before tomorrow's vote on the user charge, and the barge interests were not yet ready to give up.

Some opponents of the waterway fee were preparing amendments to Domenici's proposal that would make it unattractive to particular elements of the Senate. Sen. John C. Danforth (R-Mo.), for example, planned to introduce a plan extending the user charge to ports and harbors as well as inland routes.

Danforth could argue that what aws fair for inland routes was fair for costal shipping too.If his amendment passed, Domenici would lose dozens of votes from coastal-state senators.

Some of Domenici's adversaries placed their hopes instead on Den. Russell B. Long, the Louisiana Democrat who had led the opposition to the user charge all along. Long was famous for getting his way in the Senate maybe he could still do something, the bargemen hoped, to get his way on the waterway fee.