With the waterways toll bill due to come for debate on the floor within 24 hours, members of the U.S. Senate yesterday afternoon decided it was time to find out what the legislation was all about.

Despite hundreds of thousands of dollars worth of lobbying effort and millions of words in reports, "Dear Colleague" letters and newspaper articles, most senators who had no direct interest in the legislation had to admit to their assistants that they weren't really sure what it was they were about to vote on.

Accordingly the offices of Sen. Pete Domenici (R-N.M.), sponsor of the waterway proposal, and Sen. Russell Long (D-La.), its chief opponent, were getting calls from staff aides all over the Senate.

"I've got to write a floor memo for my guy telling him how to vote," the staffers would say. "What's the deal on this thing, anyway?"

The problem is that, until late yesterday, neither side was entirely certain what the final "deal" would be.

The basic outlines of the Domenici measures were clear. But for the past week, the New Mexican, growing increasingly nervous as the final vote approached, had been engaged in non-stop negotiations with various colleagues to design a bill that would pass the Senate.

For a number of reasons, Domenici had decided, however, that he would bring up his original bill, S-790, as a floor amendment to the omnibus rivers and harbors bill, which was scheduled for debate today.

Long, for his part, had to deal with barge industry lobbyists who were pleading with him to find some parliamentary device that would keep Domenici's bill from reaching an up or down vote on the floor.

The legislation that sparked it all would require barge lines hauling freight on inland waterways to help the federal government pay for building those waterways.

Last week, with the vote count showing his chances to be too close for comfort, Domenici set out to win over some of his adversaries.

Through innumerable phone calls and meetings with farm state senators, the New Mexican kept asking if there wasn't some way that his bill could be made palatable to farmers.

Over the weekend Domenici and two Minnesotans, Hubert Humphrey and Wendell Anderson, hit on a plan. They would establish a limit or "cap," on the barge toll that would guarantee that commodity prices would not increase more than 1 per cent because of the waterway fee.

Not until yesterday afternoon were the details resolved. But at 6 o'clock last night an excited Domenici was able to drop on the desk at the front of the Senate chamber an amendment, with 13 cosponsors, that would assure farm support for the barge toll.

Long, meanwhile, was being pressed from both sides. The barge industry, concerned that Domenici had them beat, had been asking him to find some way - even a filibuster - to prevent a vote.

But Long's allies in the Senate, Sens, John Danforth (R-Mo.) and Thomas Eagleton (D-Mo.), argued that they ought to let the bill come to a vote on the floor.

Last night, as the bill was formally brought up so debate could begin this morning, it was still not clear when - or even if - the Senate would finally vote on the waterway toll.