Negotiators for California, Nevada and the federal government that will prevent additional gambling casinos at pollution-threatened Lake Tahoe."This will save Lake Tahoe from further environmental degradation and eventually restore some of the amenities that have been lost," said Charles Warren, chairman of the President's Council on Environmental Quality.

Lake Tahoe, bisected by the California-Nevada border, is considered one of the world's purest high alpine lakes.

But the clear blue waters of the lake have been endangered by sawage and increased runoffs from developments. Automobile congestion surrounding the casinos at the lake's south shore already has contributed to significant increases in air pollution.

The agreement, scheduled for announcement today, would limit casinos to the areas they already occupy. Any expansion would have to meet strict environmental tests and the approval of the Tahoe Regional Planning Agency.

In the past this agency has been a stumbling block to environmental regulation because of a rule requiring the vote of a majority of delegations from both states before any project could be rejected. The new agreement retains the dual majority, but it is required for approval rather than rejection of any project.

The agreement, by changing the composition of the agency and expanding its membership, also will reduce the influence of local representatives from Douglas County, Nev., who traditionally have favored additional casinos.

Another key provision would provide $12.5 million in federal funds for buying up two prospective casino sites on which preliminary construction work had begun. The land would be added to the nearby Toiyabe National Forest.

However, a letter signed by the four senators from the two states agrees that no money will be spent for this purpose until the California and Nevada legislatures and Congress have ratified the new agreement, which will be presented as amendments to the existing bistate compact on Lake Tahoe.

Approval by California and by Congress is considered routine. Nevada approval, once very much in question, is regarded as likely because of the desire of Nevadans, including casinos owners at the lake, to avoid federal intervention.