From this IRS publication
Contributions made by a U.S. citizen or resident to a Canadian registered charity are treated as charitable contributions for purposes of IRC 170(c). See Article XXI, paragraph 5 and Notice 99-47. However, the charitable tax deduction is subject to two restrictions. First, a U.S. donor may use the deduction only against its Canadian source income. Second, the deduction is subject to the percentage limitations described in IRC 170(b). The percentage limitations permit a U.S. donor that made charitable contributions to a Canadian registered charity classified as a private foundation to deduct up to 30 percent of the donor’s income derived from Canada. Any excess may be carried over and deducted in subsequent taxable years. This percentage amount is raised to 50 percent if the Canadian registered charity is classified as a non-private foundation under IRC 509(a)(1) or IRC 509(a)(2). (The percentage limitation does not apply where the Canadian registered charity is a Canadian college or university at which the donor, or a member of the donor’s family, is or was enrolled (see Article XXI, paragraph 5)).