For water to freeze and change into ice, it must be cooled to its freezing point, which implies heat loss. That heat loss occurs when the ambient temperature of the air is lower than the temperature of the water.
The freezing point of fresh water is 0C; however, its maximum density is reached at 4C. From an ecological perspective, the latter point is extremely important, since the deeper water which is located under the ice except in some small lakes or ponds - does not freeze, which means that the creatures living there can survive winter under the ice. Let us take the example of a lake whose temperature decreases to 4C in the fall. Since density (the weight of the water per unit of volume) increases with the decrease in temperature, the entire lake from the surface to its bottom will reach 4C after a certain length of time. When it cools, the thin layer close to the surface of the lake becomes denser, therefore heavier, and sinks toward the bottom of the lake, and is replaced on top by warmer water which rises to the surface.
If the temperature continues to drop and dips below 4C, the layer close to the surface becomes less dense when approaching the freezing point; it then increases in volume instead of shrinking, and becomes lighter than the water below it at 4C. Therefore, that cooler layer will float on the surface and will continue to cool until ice is formed. Ice (a solid) is lighter than water (a liquid) due to its larger volume, and that is why it floats. The water located under the ice below remains at 4C, except for the layer just below the ices surface. That layer will approach the freezing point of 0C. As it reaches that temperature, it turns into ice as well, making the layer of ice on the surface even thicker. The colder it gets, the thicker and the more solid the layer of ice becomes.
If water behaved like other physical bodies, it would increase in density as it cools. If that were true, rivers, streams and lakes would be frozen from the surface right down to the bottom. In the summer, only the top layer would melt, while the deeper water could remain frozen throughout the year. In such conditions, it is obvious that aquatic life in our rivers and lakes would be impossible.

Salt water the exception to the rule
Since the freezing point is lower in salt water than in fresh water and that the density of salt water continues to rise with the decrease in its temperature, ice is formed less easily in salt water than in fresh water. Due to its properties, salt water must be cooled to a greater extent before ice can begin to form; contrary to fresh water, an entire column of water must be cooled before ice can be formed on the top.
So, what happens with salt water that is, water with 25 parts per thousand (ppt) or grams per liter is entirely different. Its maximum density is reached at a temperature below the freezing point. This means that during the cooling process, the entire column of water (at least the water with the same degree of salinity, because the deeper layers - those beyond 50 m. - are often more saline and therefore denser than the upper layers) must be cooled sufficiently before ice can begin to form on the surface.
In the Gulf of St. Lawrence, salinity can reach 35 ppt; the freezing point in that case is close to 2C and the temperature of the maximum density is even lower. However, in the tidal estuary where salinity is 20 ppt (therefore less), water freezes at 1.08C, whereas the temperature of its maximum density is 0.5C. The temperatures of the maximum density and the freezing point intersect at a salinity of 24.7 ppt. So we can see that this characteristic of water will influence the time when ice is formed in the various reaches of the river, and dictate what thickness it will attain.


Note: I would love to give credit to whomever wrote this article, but the author is unknown to me.

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