US citizens must report all of their worldwide income annually, not only the income earned in the US. You will need to file a return if meet the filing thresholds, even if the income is from Canada.
You may be able to exclude your earned income from work and/or take a credit against any US tax calculated for taxes paid to Canada on the same income. You can use whichever of these methods (or in some cases a combination of them) that gives you the best result.
Learn more about this topic here: Filing Taxes While Overseas.
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