I just downloaded the USGS temperature series for the Potomac River at Little Falls back to 1988 [as far back as it goes]. Amazing stuff.
Highest Max water temperature: 7/22/11, 35.4°C [96 F]
Highest Min water temperature: 7/24/11, 32.7°C [91 F]
Highest Mean water temperature: 7/23/11, 33.5°C [93 F]
That’s veritably bath water...