ONE OF the last things Congress did before its July recess was to give final approval to legislation creating in the heart of Idaho the 2.23 million-acre River of No Return Wilderness. But the battle that was fought over central Idaho is only a warm-up for what is scheduled to happen when congress returns.

High on the Senate's calendar is the Alaskan lands bill, the biggest, most complex and most heavily lobbied legislation in decades dealing with natural resources. We would like to think passage of the Idaho bill is a good omen, but it is still too early (even though the Alaska fight has been going on for more than three years) to guess what the Senate will do.

There are similarities between these two major land-use fights. In Idaho, the objections to the wilderness bill were that it raised the specter too much federally controlled land in the state, as well as restricting too sharply access to timber and to what are said to be major cobalt deposits. In the Alaskan debate, the major issues are identical, except that oil replaces cobalt as the scarce commodity in question.

The congressional decision in Idaho was to draw the wilderness boundaries in such a way that neither timber nor cobalt production will be reduced. The most promising cobalt deposits were left outside the new wilderness area while land that may have cobalt under it can be mined in the future if the mineral is needed.

A somewhat similar arrangement worked out for Alaskan timber and oil in the House is now, unfortunately, being described in the Senate as too big a federal land grab. The development interests in Alaska want immediate access to all potential oil lands as well as to vast forests.

The solution reached in Idaho can be a guide to settling the Alaskan dispute -- but only if those fighting over the natural riches of the Far North keep in mind something said last week by Sen. Frank Chruch, the principal proponent of the River of No Return Wilderness. The battle, he told his colleagues, "is a fight to preserve for all time a part of the vanishing American frontier." The battle lines now move from Idaho to Alaska.