In what may be the most striking sign of the start of the much discussed redevelopment of Pennsylvania Avenue, traffic will be rerouted starting this Saturday on several key downtown streets and intersections.

E Street will be closed westbound between Pennsylvania Avenue and 15th Street for about six months. Westbound E Street traffic will be diverted to Pennsylvania Avenue then south on 15ths Street.

The purpose is to permit the start of work on two principal features of the plan for the western sector of the Avenue - Pershing Park and Western Plaza.

The design for Pershing Park, on the Avenue between 14th and 15th streets calls for a cascading water-fall and pool, a memorial to Gen. John J. Pershing and an outdoor cafe. Landscaping is to be lush, trees plentiful and the entire park little changed in its dimensions from what now exists there.

The Western Plaza will run from 13th to 14th streets and will consolidate into one unbroken expanse the archipelago of traffic islands now found there.

It is to include landscaped areas at either end, and in the central section will embody a map of the L'Enfant plan of Washington, incised in granite paving.

Completion of the work is expected by the end of 1980, according to Rita Abraham, spokesman for the PADC. In the meantime, traffic patterns will be adjusted as necessary to permit work to proceed, she said.

Work is now under way, she added, to remove existing road markers and traffic signs and replace them with indicators directing motorists through the new traffic pattern.

Some utility lines in the area have already been moved in preparation for work on the plaza and square, she said.

Two Washington landmarks, neither perhaps as widely noticed as traffic patterns, already have been shifted to prepare for development of the open space.

A 50-year-old English Oak has been moved from a traffic island in front of the building to a new location in the area designated for the plaza. The tree is a gift of the French people, sent to express appreciation for American economic aid immediately after World War II.

The statue of Alexander (Boss) Shepherd also has been moved.He won his nickname as the dominant figure of the territorial government set up here in 1871. For years he stood on a pedestal on a triangle in front of the District Building. Now he has been taken to a storage area.

But he is going to come back, Abraham promised, and when he returns to the new Western Plaza, she said, he'll be back in nicer surroundings.

Count Casimir Pulaski, a Polish hero of the American Revolution, sits on a proud charger near 13th Street in what is now Pulaski Park.

The park will become part of the western plaza and the statue will remain. Pulaski will not be displaced but whether he or persons visiting him can be protected from the flocks of birds that now befoul the area, Abraham said, is a problem not yet solved.

Commercial development is also part or the overall plan to revitalize the Avenue. Seven commercial projects are listed as "active," with one building under construction at 13th and E streets NW.