Anecdotally, I know the water outflow from Lake Raystown in Pennsylvania (https://en.wikipedia.org/wiki/Raystown_Lake) is SIGNIFICANTLY colder than the lake surface is. I spent a week in the summer there one year and we spent a lot of time boating/swimming in the lake as well as tubing down the river and the days we spent tubing were much warmer air temperature but much colder water temperature.

Edit: I also couldn't find any hard numbers!