How can the freezing of surface waters make the water that is left unfrozen denser?

See below  

Step-by-step explanation:

When pure water freezes at a given temperature, the unfrozen water below does NOT become denser. It has the same density as always.

When an aqueous solution freezes, the unfrozen solution becomes denser.

That's because some of the water has been removed as ice. However, the solutes remain dissolved in a solution with a smaller liquid volume. The concentration of the solution increases, so the density increases.