The operator of Conowingo Dam on the Susquehanna River announced yesterday it has agreed to make several changes at its hydroelectric plant in hopes of replenishing American shad in Maryland.

"It's almost too good to be true," Torrey C. Brown, state secretary of natural resources, said of the decision by Philadelphia Electric Co. to provide year-round continuous flow of water past the 94-foot-high dam.

Maryland officials have been trying for nearly 12 years to reopen the Susquehanna River to the spawning runs of American shad.

The dam's construction 60 years ago had effectively blocked the shad's run.

The utility also said it would see that the water has enough oxygen in it for fish to survive and that it has agreed to consider building a permanent "lift" to help the spawning fish move upstream.

Maryland's commercial shad catch declined from 7 million pounds in 1889 to slightly over 20,000 pounds in 1979, the year before the state-imposed fishing moratorium.

A federal ruling last year ordered the building of a $3 million "demonstration" lift to scoop shad and other spawning fish from the river below the dam and deposit them in the reservoir. If the fish population thrived, as state biologists had hoped, the lift was to be replaced with a larger one.

Philadelphia Electric said it would look into building the larger permanent lift this year without first building the demonstration project.