No. A viral screenshot purported to show a Truth Social post in which President Donald Trump threatened to “turn off the tap” to Niagara Falls unless Canada paid a 100% tariff. Snopes rated the screenshot fake after finding no matching post on Trump’s Truth Social or X accounts and no coverage from major news outlets.
Niagara Falls water flow is managed under the 1950 Niagara Treaty between the United States and Canada; it is not something a U.S. president can unilaterally shut off. The fake post circulated amid a real U.S.–Canada trade dispute and echoed the style of other satirical Trump posts.
Source: Did Trump threaten to turn off Niagara Falls water unless Canada pays tariff? — Snopes