In the name of economy, the Senate voted yesterday to deny pay raises to the highest-paid federal employes after voting to go full steam-shovel ahead with the most expensive water project in U.S. history.

By vote of 78 to 13, the Senate agreed to block 9.1 percent raises scheduled to take effect Oct. 1 for the civil servants making over $51,112 a year; plus judges and members of Congress.

That amendment; added by Majority Leader Robert C. Byrd (D W.Va.) to a $12 billion energy and water-resource appropiation, is virtually the same as one passed by the House in July.

The full measure, the first of 13 major fiscal 1981 money bills to move through the Senate, was passed, 83 to 9.

Congressional sources said the cap on pay raises is likely to become law before Oct. 1 as part of one the various appropriations measures that soon will go to President Carter for signature.

Byrd estimated that the restriction on raises would save the Treasury about $100 million in fiscal 1981.

Critics of Byrd's move, noting that freezes have prevented high-level federal workers from having full cost-of-living raises since 1977, contended that the cap would drive more top talent out of federal service. But as Byrd conceded, Congress is intent on holding bureaucrats' and judges' salaries below its own, now frozen at $60,662 per year.

Among local senators, Maryland's Paul Sarbanes (D) and Charles McC. Mathias (r) and John W. Warner (R-Va.) voted against the freeze: Harry F. Byrd Jr. (Ind.-Va.) was for it.

Of the estimated 20,865 salary earners caught in the freeze, 1,500 are in the legislative and judiciary branches. The remainder are in the executive branch, basically in pay levels of Gs16, 17 and 18, and the upper reaches of Gs15.

Another 10, 000 individuals whose salaries would move above $51,112 with noral inceases during the next fiscal year also would be affected, with the cap preventing them from rising above the freeze level.

Actually, yesterday's action involves more than the 9.1 percent to which Byrd and his floor allies alluded. Without the new cap, many affected workers also would receive a 7.4 percent increase due them for the current fiscal year.

Although the pay freeze generated spirited debate, the real heat yesterday involved an effort to scuttle the Tennesse-Tombigbee Waterway, a 232-mile-long barge canal being built in Mississippi and Alabama.

Sen. John H. Chafee (R R.I.), denouncing the project as a wild boon-doggle, proposed to but $200 million from the appropriations bill's $203 million allotment for the Army Corps of Engineers canal.

But southern senators led by John C. Stennis (D-Miss.) worked feverishly for weeks and were ready for Chafee, overwhelming him on a 53-to-36 roll call vote.

No little help came from the Appropriatins Committee, on which Stennis is second-ranking Democrat, and its energy and water resources subcommittee, chaird by J. Bennett Johnston (D-La.), The committee, pushing its bill to the floor a day earlier than expected on Tuesday, took the unusual step of distributing a 43-page white paper designed to rebut charges that the canal is an unjustifield project that may cost more than $3 billion.

It also rushed off the presses a copy of hearings held on the project less than a month ago, prompted in part by a vote in June when Chafee fell just 11 votes short of stopping the project.

Johnston, Stennis and others argued that the Tenn-Tom will cost no more than $1.7 billion and accused Chafee, environmentalists and railroads -- opponents of the project -- of distorting facts.

Daniel Patrick Moynihan (D-N.Y.), a frequent critic of the project, said it would be "a Tinkertoy of a canal" that would be an economic and environmental disaster.

To Moynihan's earlier dismay, Byrd pulled off another coup -- winning a $284 million authorization for a flood control project on the Tag Fork River in his home state and then getting $6 million added to the bill to start work.

Both steps were unusual: the authorization on a money bill, which Moynihan said undercut his own water-resources subcommittees work, and the instant appropriation, which the corps was told to find from "slip-page" in other projects.