Another of those frequent border scrapes between the United States and Canada is bubbling up again, and this time it's sort of a flap between bread and caviar.

The issue is the Garrison Diversion Unit, a $1.3 billion Interior Department irrigation and water supply project creating a massive system of lakes and canals that will provide irrigation water for about 250,000 acres of North Dakota farmland.

But it isn't quite that simple.

The project would divert water, as well as predatory fish and pollution, from the Missouri River system across the Continental Divide and into a Canadian river system that leads into huge Lake Winnipeg in Manitoba Province.

The International Joint Commission, which oversees U.S.-Canadian boundary issues, warned in 1975 that the diversion "would cause significant injury to health and property in Canada"--an apparent violation of a 1909 boundary waters treaty between the two countries.

Now for the bread and caviar.

On the North Dakota side of the frontier, they want the irrigation water to help grow the wheat that makes the bread. On the Canadian side, they don't want the U.S. water for fear of what it will do to Manitoba's large commercial fishing industry, which counts whitefish roe among its aquatic delicacies.

Scientists and environmentalists here and in Canada fear that a mixing of the waters of the Missouri system with those of the Hudson Bay watershed could be disastrous to the important fishing and tourist industries that provide jobs for thousands of Canadians in Manitoba.

A delegation of Canadian lobbyists--fishermen, town officials, naturalists, tourism developers--was here last week visiting every senatorial office it could get into, handing out samples of Lake Winnipeg caviar and pleading for a halt to the Garrison project.

"We feel the International Joint Commission has been close to being violated by the U.S. determination to go ahead with Garrison," said Graham MacDonald, one of the Canadian lobbyists. "When you see citizens like us doing things here on our own, it is a sign something is not working at the top."

The Canadians were plugging for support of an amendment planned by Sens. Gordon Humphrey (R-N.H.) and William Proxmire (D-Wis.), which would strike any funding for Garrison from the fiscal 1984 energy and water development appropriations bill.

"It is old-style pork," said a Humphrey aide. "The supporters of Garrison are unwilling to discuss alternatives. We're not trying to dictate to North Dakota, but we're saying let's pause and see if there's a better way to do this."

The National Audubon Society, a chief U.S. critic of the project, has proposed an alternative, based on a study it commissioned from a Colorado engineering firm. The firm came up with a less costly and less environmentally objectionable plan that would provide farm and municipal water without intruding on the Canadian watershed.

Audubon calls the Garrison project "a gigantic giveaway" that will benefit a relative handful of farmers. "It is curious that we would be going ahead with an irrigation project to produce more wheat when the country has a huge surplus and is asking farmers to idle their cropland to reduce surpluses," said Audubon's Charlene Dougherty.

So far, on Capitol Hill, the House has voted for caviar and the Senate has stuck to wheat, mostly at the insistence of North Dakota Sens. Mark Andrews (R) and Quentin N. Burdick (D), who support the Reagan administration's request for $22.3 million to continue Garrison construction, now only 15 percent complete.

The House, by a 100-vote margin, voted last winter for a proposal by Rep. Silvio O. Conte (R-Mass.) to stop Garrison funding. A House-Senate conference, however, put $4 million back in the bill to carry the project through this fiscal year.

This time around, the House appropriated no money for Garrison and the focus now is on the Senate Appropriations Committee, where Andrews and Burdick, both members, will move this week for inclusion of the $22.3 million.

"We're sure the subcommittee will put the money in its bill," said an Andrews aide. A vote for bread, that is.