The northern U.S. border with Canada is 4,000 miles long, largely unfenced, unmarked and unguarded. Much of the terrain is remote and rugged, making it difficult for border agents to monitor using traditional methods.

The challenging landscape and lack of manpower may have contributed to the results of a recent Government Accountability Office study suggesting less than 1 percent of the northern border had an acceptable level of security in 2010. The study also found the Border Patrol was aware of all illegal crossings on only 25 percent of the 4,000 miles.

While not commenting specifically on the GAO report, Border Patrol agents tell Fox News they're covering a lot more ground with a lot less manpower than their fellow agents in the south.

The southern border is about half as long, at 1,952 miles, but there are 18,000 agents assigned to California, Texas, Arizona and New Mexico.

That's nine agents per mile. There are 2,200 agents assigned to the north, the equivalent of one agent for every two miles of territory, forcing the federal government to depend on state and local officers for backup.

“The Border Patrol can’t do the job alone up here," says Chief Patrol Agent John Pfeifer, who runs the Swanton Sector in Massena, N.Y. "We rely on those other agencies to help us secure the border."

Border agents took Fox News on an exclusive ride-along on Snowcat snowmobiles along the frozen St. Lawrence River to demonstrate just how difficult and challenging the landscape can be. The morning ride took place in arctic conditions with snow and high winds bringing the "feels like" temperature to near zero. Even the Snowcats had trouble in some of the thicker drifts.

Dressed in a thick snowsuit, warm winter boots and helmet, Agent Glenn Pickering told Fox he looks for "anything out of place… any snowmobiles coming across the river, anything that looks out of the ordinary.”

He says it's a game of cat-and-mouse. Agents know the smugglers are watching them too, using speed boats in warmer months and lightning-fast snowmobiles in the winter, crossing the iced-over waters into America, often going right through tribal land.

According to the Justice Department, up to 20 percent of the highest-potency marijuana produced in Canada every year is smuggled through one 10-mile stretch of the border, the Saint Regis Mohawk Indian Reservation.

Several smuggling rings have been busted on the reservation in recent years, according to U.S. officials, who coordinate patrols on tribal territory with the Mohawk's own police force while avoiding confrontations with some members who resent any intrusion on their land.

Signs posted along the major public highway running through the Akwesasne Mohawk Indian Reservation say "This is Mohawk Land, not NYS Land!!!" and "Yes, 'terrorists' come thru Akwesasne. They are N.Y.S.P., Border Patrol, A.T.F., F.B.I., Etc. Etc.!!!"

Mohawk Police Chief Andy Thomas admits, "there are some traditional folks who believe that outside agencies should not have a part our territory." But, he says, "It's been that way forever."

He also says the tribe routinely cooperates with authorities, is doing the best it can to stop illegal activity and has been unfairly criticized in the media.

"There is no doubt we do have illegal drug activity coming through the Mohawk Territory but we are not exclusive in that," Thomas says. "It's happening throughout the northern border."

Pfeifer, the Border Patrol chief, agrees the troubles extend well beyond Mohawk land and says the agency's primary focus is on keeping America safe from the most serious threats.

"That's our primary mission, to protect the American public" he says. "It's great to be out there seizing drugs and arresting people with all this contraband. ... But at the end of the day, our job is to stop that person that's coming to the United States to do us harm."