The Metropolitan District Commission said Thursday it will fill in "slightly" more than one acre of the Charles River to replace recreation area destroyed by the Boylston St. underpass.

The MDC, apparently reacting to vocal opposition to the three projected Memorial Drive underpasses, also adopted plans to save all but seven of the sycamore trees lining the Drive. Construction on the underpasses, which will be built at River St. and Western Ave. in addition to Boylton St., is scheduled to begin next March.

Despite the new plans, opponents of the underpasses continued to insist that the traffic situation does not warrant the underpasses and that their cost, more than $7 million, is a waste of money.

They charged that the construction of the underpasses will eventually lead to the widening of all Memorial Drive and the distraction of irreplaceable recreation area along the Charles.

Transplant 130 Trees

According to the new plans, the MDC will transplant 130 trees, including 35 sycamores, that would have been destroyed by the underpasses. Seven sycamore trees, deemed too large to be relocated economically, will be replaced by younger trees.

The filling in of the Charles will not be begun until the completion of the Charles River Dam, which will be started next year. No part of the river can be filled in until the dam is finished because of present flood control regulations.

Moriece and Gray, Cambridge landscape architects, prepared a study of the Charles River area along Memorial Drive and the MDC adopted its recommendations.

Accordingly, the MDC will also construct a pedestrian overpass near Weeks Bridge "to make the crossing of Memorial Drive safer," and also "beautify" the riverbank between River St. and Western Ave.

Edward L. Bernays, co-founder of the Citizens Emergency Committee to Save Memorial Drive, said the new proposals were merely an attempt "to take attention away from the main concern: the destruction of Memorial Drive."

He quoted a portion of an original traffic report, prepared by the Boston engineering firm Jackson & Moreland for the MDC, as evidence that the underpasses would probably require the widening of all Memorial Drive.

The relevant section said: "To achieve maximum benefits from grade separation [underpasses], it will be necessary to match capacities along the length of Memorial Drive." Bernays said this meant that the Drive, presently 40 ft. wide, would have to be widened to 52 ft., the width of roadways for the underpasses.

Bernays also declared that many experts feel that the underpasses are unnecessary and that the opening of the Massachusetts Turnpike's Cambridge exit had eased traffic on Memorial Drive. Last fall the Cambridge Planning Board opposed the underpasses and proposed a $100,000 plan of automated traffic control, which, it claimed, would largely solve rush-hour congestion on the Drive