Since Canada pulled off the string rescue of American hostages from Iran, there has been a feeling that the United States owes its northern neighbor special debt.

But if recent actions by the U.S. Senate are a true indicator, the friendly relationship between the two countries seems to have fallen on difficult times.

Item. An East Coast fishing treaty between the two countries, approved by Canada, languishes in the Senate in large part because two senators think it is unfair to fishermen from their home states. The treaty sets boundaries and quotas for American and Canadian fishermen.

Item. Although Canada had repeatedly raised environmental concerns, the Senate last week handily adopted a bill requiring 80 U.S. oil-burning electric power plants to convert to coal without new pollution controls. Canadians fear intensified acidic air pollution as a result.

Item. Even though critics said the action would violate a U.S.-Canada boundary treaty, the Senate agreed Friday to provide go-ahead money for a bitterly disputed North Dakota irrigation project that alarms Canadians.

All three issues have brought recent official Canadian expressions of concern to the Carter administration and Senate leaders, with little or no positive reaction.

Two weeks ago, for example, Prime Minister Pierre Trudeau raised the fishing treaty problems with President Carter during their meeting in Venice.

Last month, Mark MacGuigan, secretary of state for external affairs, announced that Canada would allow overfishing of the rich Georges Bank zone because of the fishing treaty hangups.

In a statement issued in Ottawa, MacGuigan linked the fishing dispute to the solution of other problems between Canada and the United States.

"If, in future negotiations, Canada were to withhold concessions -- in the expectation that we would face further negotiations when an agreement reaches the Senate -- we might not be able to go beyond the first stage and reach a signed agreement," he said.

Part of the problem, as MacGuigan was noting, is the extra dimension of political log-rolling and regional politics that affects virtually everything the Senate does.

A classic example is the dispute over the Garrison Diversion Project, a massive U.S. irrigation scheme for North Dakota that would divert Missouri River waters into Canada's Hudson Bay Drainage basin.

The administration has withheld funding, the House has withheld funding, but North Dakota's senators, infleuntial as members of the Appropriations Committee, have won more money for Garrison.

It wasn't much -- just under $10 million -- but it was enough to arouse Canadian fears that the project would devastate the fishing and tourism industries of Manitoba.

North dakota Sens. Milton R. Young (R) and Quentin N. Burdick (D) assured the Senate that none of the money would be used on parts of the project that would affect Canada.

Canadians in Manitoba and American critics from the National Audubon Society argued that unless the project is built as originally authorized -- sending U.S. waters into Canada -- it would be illegal or useless, or both.

This view coincided with the International Joint Commission, a bilateral group, which held in 1977 that Garrison, as authorized, would violate the two countries' 70-year-old boundary treaty.

The Canadians fear pollution of three major river systems in Manitoba, tainting of municipal water supplies and transfer of Missouri fish and fish diseases into Canadian streams and lakes.

But Young is retiring this year, and the Senate was in no mood to overrule a man who has devoted much of his career to seeing Garrison completed.

"What in the world has to happen to get your attention?" a Canadian diplomat asked. "Does Manitoba have to declare war?"

Similar Canadian emotions have engulged the coal-conversion and fishing treaty issues, both dependent on action by the Senate.

Canada and the Carter administration spent 18 months hammering out the East Coast fishing agreement. Canada's signing assured its participation. But in the United States, the agreement had to be submitted to the Senate for approval. The Foreign Relations Committee held two days of hearings last spring -- nearly a year after the signing -- and then put the issue on a back burner.

Principal objections to the treaty by Sens. Clairborne Pell (D-R.I.) and Edward M. Kennedy (D-Mass.), as well as a parliamentary block created by the pending SALT II agreement, have stopped the fisheries pact.

Pell, Kennedy and others from the New England states contend the treaty is unfair to their fishermen.

Last month, when it became apparent that U.S. fishermen were going beyond quota agreements on scallops, Canada announced that its haddock, cod and flounder fishermen would no longer abide by the quota terms.

"If the total catch is to be kept within safe limits in the Georges Bank, the fishing quotas of the U.s. fishermen must now be reduced," said Romeo LeBlanc, Canada's minister of Fisheries and Oceans.

"Failure to do so will create an overfishing situation on stocks of greater interest to U.S. fishermen than to Canadians."

A Foreign Relations Committee aide agreed with Canadian assessments that the treaty dispute could lead to more severe complications between the two countries.

"It could get very ugly off the East Coast, with people taking potshots at one another. It could get hairy, but there is no way the full Senate can get to the treaty under existing conditions," he said.

Somewhat more complicated and technical is the acid rain dispute which has set off alarm bells on both sides of the international frontier.

As coal combustion in power plant boilers is increased, without corresponding tightening of air pollution control requirements, there is a fear that sulfur and nitrogen oxide fallout will further poison lakes and streams throughout the Northeast.

Power plants in the United States and Canada already have been identified as sources of the acid fallout that has rendered many bodies of water unable to sustain aquatic life.

Diplomats here and in Ottawa make the point that issues seen in the United States as mere brushfires are viewed by Canadians as conflagrations affecting border relations.

House Majority Leader Jim Wright (D-Tex.) touched the same theme recently, telling his collagues that U.S. indecision on a joint trans-Canada pipeline venture is raising new concerns about U.S. promises.

Canadians are fearful that if they proceed with construction of their share of the pipeline, a reversal of U.S. support could leave them with a long chuck of non-functional machinery.

"There is a great deal of comment at home about this and the other issues," a Canadian official here said. "We seem to be going nowhere . . . there is a deep sentiment of frustration in the Canadian government."