There are a number of ways in which these two countries could resolve this dispute. Most of them would mainly involve efforts on the part of the United States since it is the party that is causing the problem. However, Canada might do some things to show good faith and to encourage the US to be more interested in remedying the problem.
One approach to the problem would be to find ways for the US to stop creating so much pollution. This is largely incumbent on the US, but Canada might donate some money towards researching ways to reduce pollution. These efforts could include finding cleaner energy sources. They could also include the creation or implementation of technology to scrub the emissions from plants that burn coal.
The other main approach would be to attack the effects of the problem. This is, of course, less attractive of a solution. However, it could be used in conjunction with efforts to reduce pollution. This approach could take the form of US donations to Canada to be used to remedy the effects (such as acid rain in lakes) of the pollution that comes from the US.