[dredmorbius]

It would of course depend on the river and its own temperature.

Keep in mind that we're now seeing sea surface temperatures above 38°C (100°F).

If the river itself is slow-moving and is in an area of prolonged heat, it will also be warm.

The areas most prone to high temperatures often also have little surface water, or at least, not enough to shelter all persons affected.

Water refuges being too warm for human survival is an early plot point in Kim Stanley Robinson's Ministry for the Future.