Some 260 miles north of Toronto, the still waters of Lumsden Lake are fit for a picture post card. Shrouded among the quartzite of the La Cloche mountains above Georgian Bay's north shore, Lumsden's waters are a brilliant turquoise. Yet the life in Lumsden Lake, like dozens of other lovely lakes in this part of Canada, is slowly disappearing - a victim of acid rain.
''There was a time the waters weren't so brilliant,'' says Harry Thorsen, who lives on the lake's eastern shore. ''It was murky then, a little scary at first glance, for you couldn't see the bottom through its dark green waters. But it was clean. And the fishing was excellent. There aren't any fish left.''
Many of Canada's thousands of lakes and streams, once alive with fish and other living organisms, are in trouble. Everyone here knows the cause. And many Canadians tend to blame the problem on the United States.
Millions of tons of sulfur and nitrogen smoke belch out of smelters, coal-fired power plants, and motor-vehicle engines in northeastern US - and then blow across Canada, falling to earth as corrosive sulfuric and nitric acid.
Angry denunciations of the US have become commonplace. Yet, that conventional wisdom is giving way today to a realization that Canada, too, is responsible for acid rain, that Canadian factories do the same as factories on the US side of the border. The giant smokestacks at Sudbury, 350 miles north of Toronto, are every bit as responsible for acid rain as those at Cleveland or Pittsburgh. Former Canadian Environment Minister Romeo LeBlanc warns that acid rain is ''an international time bomb.'' To resolve the problem, he argues, US-Canadian teamwork is needed.
There is much hope here the two nations can muster that teamwork. Canadians welcome the Reagan administration's recent announcement that it wants to meet the issue ''head-on.'' Environment Minister John Roberts says the US ''clearly believes there is an urgent problem (regarding acid rain).''
Canadians consider acid rain a serious problem - perhaps the dominant one - in relations between the two countries. There are, however, numerous other problems troubling the relationship. Many are centered on economic issues: trade barriers, US investment in Canada, and ocean fishing.
Take fishing. In British Columbia, where the cool Pacific waters are home to salmon, Canadians complain the catch is dropping dramatically, threatened by US overfishing. Alan Meadows, a genial Canadian fisherman whose 40-foot trawler goes out for 10 days at a time to bring in the prized chinook salmon catch, complains bitterly that the US refuses to put limits on salmon fishing. ''The result will be the depletion of our salmon,'' he says.
A salmon war also could result. Canadians could start fishing in US Pacific waters, netting a bigger share of the pink and sockeye salmon in these waters, while Alaskan fishermen may move more vigorously into Canadian waters to net the chinook salmon.
''With no treaty,'' says Mr. Meadows, ''the two countries are going to find themselves in difficulty.''
Canadians realize that for the US, such issues seem relatively unimportant.
Yet for Canadians, salmon and acid rain are extremely urgent issues, and they want them addressed.
Living next to the US is perhaps the essential element in the relationship. It is not easy, say Canadians, to live next door to the world's mightiest power, particularly when that power tends to take you for granted. Canadians bridle at that attitude. They also do not appreciate the oft-expressed US view that Canadians are just like their US cousins.
After all, reason those in the US, except for French-speaking Quebec, North Americans on both sides of the border speak the same language, produce much the same products, have compatable economies and living styles, enjoy the benefits of two of the few democracies in the world - in short are very much alike. They are also each other's largest trading partners.
Two-way trade came to $102.80 billion in 1982, Canada's exports to the US accounting for 17 percent of all US imports, with US exports bound for Canada filling a whopping 70 percent of Canada's market.
How to maintain independence is the basic Canadian problem - especially when in most things, Canada feels comfortable with its southern neighbor and when that neighbor is its largest trading partner.
Former Trade Minister Mitchell Sharp puts it this way: ''I think there is a tendency for the United States to overlook Canada in matters of economic and foreign policy. It's easy to say Canada is a nice country, that it doesn't cause much trouble. But benign neglect can sometimes lead to unfortunate consequences.''
Canadians remain alarmed about what they regard as growing foreign control of their economy. The major threat in this obviously comes from the US. From a low of $3.5 billion in 1950, US investment in Canada totaled just under $50 billion in 1982.
''That simply is too much in a small economy like that of Canada,'' says banker Read Andrews in Toronto. ''It is as if your arm and leg really didn't belong to you, but belonged to someone else. Is it any wonder we complain? And worry?''
It does little good to point out that there is also large Canadian ownership in the US - that, for example, a good half of the new industrial and office construction in Houston is in Canadian hands, that Canadians jostle with their US cousins in the New York real-estate market, that Canadian food chains dominate the supermarket system in parts of the US Northeast, that Canadian banks are major competitors in some parts of the US and compete with US banks elsewhere.
The list of Canadian intrusions into the US market is long.
But the list of US intrusions into the Canadian market is longer, and also more apparent because of the relatively smaller size of the Canadian market. The US population at 232 million dwarfs Canada's of 24 million. The 1982 gross national product of the US at $3.06 trillion also dwarfs Canada's at $283 billion.
''Why should the US therefore be concerned about us?'' asks banker Andrews. ''Because we are basically loyal friends and good neighbors. Because the United States needs that loyalty and that friendship.''
He, like so many other Canadians, is encouraged by the current efforts by Washington and Ottawa to confer on the acid rain issue. It could be the harbinger for progress on other issues - and to Canadians suggest that the US is aware of Canada's interests.