President Reagan's recent choice of William Penn Mott as head of the National Park Service drew a round of applause from conservation groups, but many of the same environmentalists are shifting uncomfortably in their seats at word of the next act: the selection of William P. Horn to be assistant interior secretary for fish, wildlife and parks, making him Mott's superior.

As former secretary James G. Watt's deputy undersecretary in charge of Alaska lands, Horn didn't endear himself to conservationists. Under his hand, for example, the department blessed a proposed land swap that would have allowed Atlantic Richfield Co. (ARCO) to build an air base on St. Matthews Island, a wilderness and wildlife sanctuary in the Bering Sea.

Horn's reputation with the wilderness lobby goes back even farther, however. As an aide to Rep. Don Young (R-Alaska) during the Alaska land battles, Horn regularly butted heads with conservation interests over how much of the state should be left pristine and how much opened to development.

Conservationists contend that Horn carried his prodevelopment bias -- and his intimate knowledge of the Alaska Lands Act -- into Interior, where he set about interpreting some of the act's murkier provisions to the benefit of developers.

Still, conservation groups appear divided over how strongly to oppose Horn's elevation to an assistant secretary's slot.

In his six months at Interior, Secretary Donald Hodel has made a point of meeting with conservationists to hear their views. Some groups are hesitant to risk losing that access by being too confrontational on political appointments -- especially since the Mott appointment was widely regarded as a victory for conservationists.

"Our position is we'd like to give him a chance," James DeSantis of the National Wildlife Federation said of Horn. "We have a few mild reservations, based on his association with Watt and Don Young. But from what we know, he's been accessible for the most part."

Other environmental groups are not likely to be so sanguine. "It looks like it's one for the environmentalists on the park service director and one for the developers on the park service director's boss," said Tim Mahoney of the Sierra Club.

Some are already expecting lively internal battles at Interior between Mott, a longtime parks veteran who served Reagan in California, and the much younger and less-experienced Horn.

DIVERSIONARY TACTICS . . . A moment, please, to recap the saga of the Garrison Diversion project in North Dakota. When last we left this long-running water, (1) a special commission had recommended that the project be scaled back considerably in size, purpose and cost; (2) a compromise between environmentalists and state officials had collapsed on the eve of congressional consideration, and (3) everybody went back to the drawing boards.

Except, it appears, the Bureau of Reclamation. The bureau is proceeding with construction plans for the federal irrigation project, one of the largest under development, and expects to sign two work contracts next month for more than $18 million.

According to Interior officials, the work would fall within the scope of the special commission's recommendations and would be paid for with previously appropriated funds. But the move drew an angry response from Rep. George Miller (D-Calif.), chairman of the House Interior subcommittee on water and power resources, who warned Hodel in a letter last week that Garrison's future is far from certain and proceeding with the contracts "could lead to a multimillion-dollar financial loss for taxpayers."

An aide said Miller is willing to await the outcome of a new round of negotiations but isn't willing to let the bureau "foreclose the option" of scuttling the project altogether.