A merger combining Columbia Federal Savings and Loan Association, Washington's third-largest S&L, and First Federal Savings and Loan Association of Washington, the city's fifth largest, became effective yesterday.

The combined institutions will open for business this morning under the name of Columbia First Federal Savings and Loan Association.

The merger, which makes Columbia First Federal the second-largest S&L in the Washington metropolitan area, was approved by the Federal Home Loan Bank Board last month.

Total assets of the new entity equal more than $940 million, and reserves will exceed $62 million, almost $28 million above regulatory requirements. Perpetual American Federal Savings and Loan Association is the largest S&L in the Washington area, following a merger last year. Perpetual's assets at year's end topped $1.4 billion.

A wave of consolidations is sweeping the thrift industry, as S&Ls seek the financial muscle to compete with commercial banks. In Virginia, during the past fortnight, two major S&l mergers have been announced: First Federal of Arlington and Arlington-Fairfax, to form the largest in the state with assets of $768 million; and Virginia First of Petersburg with First Financial of Virginia (Washington-Lee S&L) to form the second-largest with assets of $600 million.

The combination yesterday of Columbia and First Federal in D.C. will result in 15 Columbia First offices, 14 in the District and one Maryland branch in Bethesda, opened before interstate branching laws prohibited further offices across the District-Maryland line. The merged association's main office is 730 11th St. NW.

Clarence E. Kefauver Jr., chairman of Columbia Federal, is the new chairman of the board of Columbia First, effective today.

T. William Blumenauer Jr., president and chief executive officer of Columbia Federal, assumes the same positions for Columbia First. Melton I. Baldinger, chairman of First National, becomes chairman emeritus of the board of Columbia First. Dewitt T. Hartwell, president of First Federal, becomes executive vice president and chief administrative officer of Columbia First.Loring E. Tilton, executive vice president of First Federal, becomes a senior vice president of Columbia First.