MIGHT THE Chesapeake Bay -- considered North America's largest and most biologically diverse estuary -- be clean enough in 10 years to surpass standards in the Clean Water Act? The recipe for a certifiably healthy watershed is charted in the latest regional agreement signed by state, federal and other officials to build on the progress made in the last 12 years.

But one key ingredient is missing. While all parties agree that controlling growth is critical, Virginia Gov. Gilmore is objecting to the most important specific proviso agreed to by the other five signatories: that they cut the annual loss of forest and farm land to development by at least 30 percent.

As approved for public comment, the document contains a large asterisk noting that "five of the six" parties endorse the land-use commitment. They are Maryland Gov. Glendening, District Mayor Williams, Pennsylvania Gov. Tom Ridge, U.S. Environmental Protection Agency Administrator Carol Browner and Rep. Anthony Hershey of Pennsylvania, chairman of the Chesapeake Bay Commission. John Paul Woodley, Virginia's secretary of natural resources, has argued that land-use planning powers rest with local governments and that Gov. Gilmore has no power to promise a numeric reduction -- though it was Mr. Gilmore himself who opposed efforts in the last Virginia General Assembly session to give those tools to the counties.

Besides, Mr. Woodley said, Virginia's rate of development overall -- as a percentage of total land area -- is lower than that of Maryland and Pennsylvania. This too is dubious. The Chesapeake watershed is a specific interstate region that must have specific, uniform land-use policies to protect and restore resources as well as water quality. Virginia's cooperation need not upset other balances of powers within the state.

The agreement lists other goals: to restore passages for migratory fish along 1,300-plus miles of blocked river by 2003; to establish a harvest target for blue crabs by 2001; to develop regulatory programs for achieving a no-net loss of wetlands acreage; and to increase the oyster population tenfold. Participants also committed to establishing "no discharge zones" for boaters by 2003, and to increasing waste pump-out facilities by 50 percent by 2010.

Since the first agreement in 1983, the initiatives endorsed by the six parties have produced significant results. But more than 300 years of increasing populations and expanding development keep up the pressure for protective measures that, like the bay itself, transcend jurisdictional limits.