As winter approaches the water cools until finally the temperature of the surface water becomes the same or more nearly the same as the bottom water. Then the wind exerts a deeper influence and the surface and deep water will mix. During this process vertical currents may be produced. This mixing continues until all the water has been thoroughly mixed and until the temperature of the surface water goes below the point of maximum density which occurs at 4° C. The cold water accumulates on the surface where ice is often formed. In the spring the process is just reversed. The fall mixing of the water is much more thorough and intense than the spring mixing. The changes brought by this mixing are obvious. The surface water contains a large amount of oxygen. The bottom water contains much less oxygen but a great quantity of decomposed products. The oxygen is carried to the bottom and there oxidizes and neutralizes some of the products of decomposition. The =sources of impurities= in surface water are various. One of the most menacing sources is from the sewage which is carried from the centers of population. In the rivers it is the sewage of the towns and cities situated above. In the lakes it is usually the sewage of the city itself that is likely to pollute its own water supply. Under the present conditions it seems that it is easier to purify the water supply than to purify the sewage or provide some other method for its disposal. Naturally the water of rivers is purest near the source. The reason for this is that the rivers form a natural drainage for the land through which they flow. The waste products from every process carried on in that territory find their way into the streams as a natural result of the law of gravity. The water will be found to contain large amounts of mineral and organic matter after passing through populated and cultivated areas. Purification of rivers is considered under Water Purification. Ground or Subsoil Water In the consideration of the three sources of water supply it is difficult to draw definite lines of demarcation since the rain and snow water soon becomes surface water and the surface water may percolate through the ground and become subsoil water. =Well water and spring water are classed= as ground or subsoil water. From a hygienic standpoint water obtained from wells or springs forms a very reliable and satisfactory source of supply.