I've never noticed that water starts to freeze at anything over 32F, but water, even pure water, can exist in liquid form below 32F. It is called supercooled and it is found in nature mostly at altitude in the atmosphere, not to my knowledge in rivers. It is a problem for aviation as it can be a condition that allows for airframe ice to form. I'll spare you all the gory details of how I know this but suffice it to say that I'll never forget an early spring trip into Denver one year when ice formed on my Piper enough to cause the ATC at Stapleton enough concern to have them dispatch a county helicopter to look for my wreckage. (The exact wording was something to the effect "Heading so-and-so, listen for an ELT", an emergency locator transmitter that goes off when you crash) Since that time I have made it a point to become a minor expert on airframe ice formation and how to avoid it.