The State Department is raising serious questions about the environmental consequences of the proposed Panama Canal treaties even as the Carter administration is intensively seeking Senate ratification of the pacts.

State's draft environmental impact statement on the treaties, which is required under U.S. law, suggests that eventual Panamanian control of the Canal Zone could mean deterioration of the watershed - crucial to operation of the locks.

Pressures of deforestation and population are already a threat to the watershed and measures are necessary to pervent them from mounting in the future, according to the document now circulating for comment.

Pointing out that the treaties do seek to protect the environment, and that a supplementary $10 million U.S. loan is proposed, the draft statement says that nevertheless "the forests and associated ecosystem in the Canal Zone could disappear" should these measures fail.

Ships passing passing through the canal's locks cross Gatun Lake, which also gathers the massive quantities of water needed to serve the locks on the Caribbean and Pacific ends of the 51-mile route.

The State Department draft finds that forests lying within the 10-mile-wide Canal Zone, now controlled by the United States, are virtually all that remain in the Panamanian watershed.

"There has been extensive deforestation in the areas adjacent to the Canal Zone and there are population and economic pressures for further exploitation of the forest areas.

"By contrast, because of restrictions on public access to the Canal Zone watershed and defense areas, much of the Canal Zone is now an island of forest in the midst of a generally cleared countryside.

The State Department declares that it is in Panama's interest to prevent further encroachment on the forests and that it is Panama's stated intention to do so. But it also notes that Panama does not foresee further deforestation as likely.

Panamanian agencies "have plans for the creation of forest reserves and parks, for reforestation and fore resettling squatters moved out of the Canal watershed areas," the draft states, recommending high-priority approval for a U.S. aid mission project to strengthen the institutions that would carry out the plans.

The treaty authorizes U.S. measures to protect the canal's water sources until the year 2000. After the treaty expires, however, water management would be wholly in Panamanian hands.

In offering alternatives to the proposed treaty, as required by the 1969 legislation calling for the impact statement, the State Department brings forth essentially only one option: "Changes in the present draft treaty or a new treaty with stronger environmental provisions. It finds that option unnecessary and perhaps impossible politically.

The hastily assembled impact draft contains 42 pages plus an appendix of earlier studies indicating that a possible new sea-level canal would have some destructive ecological impact.

The State Department initially obtained permission from the Council on Environmental Quality to shorten by 15 days the stautory 45-day period for public comment on the draft. As a result of public interest, the full 45-day period was reinstated, however, and is to run until Oct. 13.