Those of you who are already fantasizing a warm spring of sun-bathing and frisbee-playing by the Charles River had better forget it-at least for this year.

An MDC project designed to stop pollution of the Charles by constructing a relief sewage system is scheduled to begin early this Spring. The digging will extend from Pleasant Street, halfway between Peabody Terrace and the B.U. Bridge, to Plympton Street along the river. It will not be completed until the spring of 1972.

The relief sewage system, which will be built parallel to existing pipes, will eliminate overflow into the river. Together the sewers will carry 15 times the present dry weather flow. During rainstorms there are now at least six overflowing outlets in the Harvard area draining raw sewage into the Charles.

An MDC spokesman termed the Pleasant Street to Plympton Street digging as "part of a ten-year comprehensive plan to expand sewage capacity to alleviate the sewage problem in Cambridge and reduce the amount of combined storm and sewage drainage."

The North Charles Relief Sewer Project is the second part of a three-phase construction program stretching three miles along the river. The first phase, running from the B.U. Bridge to Pleasant Street, has already been completed.

Construction in the Harvard area will cost approximately $3 million, of which the City of Cambridge pays $27,000. The remainder will be collected from 41 other towns and cities in the area.

At some point, the construction work will move onto Memorial Drive near River Street. "It is hoped that we can do this without seriously affecting traffic patterns, probably over the weekend," a spokesman for the MDC said.