The Potomac River, which flows through the US capital Washington, hit a record high temperature of 94 degrees Fahrenheit (34 degrees Celsius) over the weekend - as warm as bathwater - following a major heat wave.
The previous highs came in the summers of 2011 and 2012, though record keeping began only in 2007.
On Sunday, the temperature exceeded previous records by half a degree Fahrenheit near the Little Falls rapids, upstream from the US capital, peaking at 93.7 degrees Fahrenheit at four feet (1.2 meters) from the shore, halfway between the surface and the bottom, according to the US Geological Survey.
The United States sweltered in dangerously hot weather over the weekend, with major cities including New York, Philadelphia and Washington broiling in temperatures that rose into triple digits.
US media blamed the heat wave for at least six deaths, including a hiker who had been found unconscious on a trail outside Washington on Saturday and two people who died earlier in the week in the eastern state of Maryland.