TO THE MOHAWK NATION, BOUNDARIES DO NOT EXIST

To hear Vaughan Aldrich tell it, he and the 7,000 other Mohawk Indians who live here form a sovereign nation whose territory just happens to straddle the border between the United States and Canada.

''My mother always told me never to pay attention to the border between these, what shall we say, these more powerful entities,'' said the Cornell- trained lawyer. He lives and works in Hogansburg, a hamlet on the United States side of the reservation served by the American ZIP code 13655. Just a few hundred yards from his house down an unobstructed two-lane road lies this village with its Canadian post office and its own code, H0M 1A0.

''That border was made by white people,'' Mr. Aldrich said. ''It has nothing to do with us. We are one tribe and we are certainly a sovereign nation. But you know, these days no sovereignty is absolute - not even that of the U.S. - and I guess you'd have to say that ours might be more circumscribed than most.''

That may be so, but as a consequence of their geography the people of St. Regis assert their independence every day. For one thing they do not have to pay the $2 toll on the bridge over the St. Lawrence that spans their reservation and coincidentally links the United States and Canada. They use a separate lane at the toll booths and they also whisk through the customs checks waving their identity cards at the United States and Canadian inspectors, often with a proud flourish. Tax and Legal Breaks

They have tax exemptions for income generated on the reservation and according to Chief Lawrence Pyke, one of their elected leaders, the police, at least from New York, can arrest residents only for the ''14 most serious crimes.'' In other offenses they have to be invited in by tribal authorities.

But if there are benefits that flow from geography there are also bizarre complications.

''Just think of it, we are surrounded by five major jurisdictions,'' said Mr. Aldrich, the lawyer. ''We have the two federal countries, the U.S. and Canada, but we also have New York State, and the provinces of Ontario and Quebec. For our phones we have three different area codes: 613, 514 and 518. Sometimes a simple thing like getting a bank loan or sending your kid to school can seem like a matter for the International Court in The Hague.''

In fact there are many children who live on the United States side of the border who go to Canadian schools. There are also children from the Canadian side of the reservation who are in United States head start programs. Competing Government Bodies

And if being encircled by five jurisdictions is not complicated enough, there are also three often competing bodies of self-government on the reservation. Chief Pyke is one of three men who serve on one of these, the St. Regis Mohawk Tribal Council, which has its headquarters in the community building in Hogansburg.

''We represent the families of the 2,234 adults who vote for us,'' said the Chief, who like most St. Regis Mohawk men has worked as an ironworker putting up bridges and the skeletons of skyscrapers from Montreal to Houston. ''We are involved with overseeing funding programs from Washington and Albany,'' he added. ''We have a clinic, administer education programs from nursery to college, have a credit union and we used to have our own police force.''

''Over there,'' he said, pointing toward the village of St. Regis, ''is the Band Council, which is elected by people on another roll. Those people look to Ottawa and they have their own programs.''

Technically, said Chief Pyke, a resident of the reservation cannot vote for both councils but there is nothing to prevent people from going from one voting roll to the other, or in effect switching from being Canadian-leaning sovereign Mohawks to become United States-leaning sovereign Mohawks or vice versa. Ottawa Seeks Charter Bargain

''It doesn't matter which side of the border anyone lives on,'' said Chief Pyke. ''You can live over there and be with us or you can live over here and be with them. In fact, right now they've opened their rolls and I heard some people were interested in switching because of a Canadian program that gives you up to $18,000 to build a new house.'' He said slightly more people voted for the Band Council than for his tribal council.

That balance may be altered by events in Ottawa. For the last two years the Canadian Government has been seeking to strike a constitutional bargain with its native people over what are termed aboriginal rights.

In a succession of conferences Prime Minister Pierre Elliott Trudeau has brought together Indian and Eskimo leaders with provincial premiers in the hope of developing a still amorphous concept of self-government for native people. The so far elusive formula would, according to Mr. Trudeau, ideally establish a third level of government beyond federal or provincial juristictions for native people. Presumably they would gain clear title on land claims and in exchange would endorse Canada's Constitution.

Lawrence Francis, the chief of the Canadian band council, shuns reporters, but Mr. Aldrich, the lawyer who represents the Canadian Band in many suits, was hospitable and forthcoming.

''Right now we have suits against Reynolds Metals and Alcoa asking for $50 million in damages for having ruined health, life and agriculture on Cornwall Island with their fluoride emmissions,'' said the lawyer. ''We have other suits for the return of 11,642 acres of our land including an island where the New York State Power Authority is operating the Robert Moses generating plant. And we have have some land claims in Canada, all attempts to regain sovereignty.''