The last case is that of a large river, with a drainage area of over 11,000 square miles. The observations extended over a full year. In this period there were sixteen floods, each lasting from one to six days, and the sum of the turbidities for the one tenth of the whole number of days when the water was muddiest is 45 per cent of the aggregate turbidities for the year. The floods occurred on an average of once in 22 days, and the average duration was two and one half days. The results are very striking as showing that a very large proportion of the mud is carried by the water in flood flows of comparatively short duration. They also show that in small streams the proportion of mud in the flood-flows is greater, and the average duration of floods is shorter, than in larger streams. In other words, the differences between flood- and low-water flows are greatest in small streams, and gradually become less as the size of the stream increases. When a stream is used for water-works purposes in the usual way, a certain quantity of water is taken from the stream each day, which quantity is nearly constant, and is not dependent upon the condition of the stream, or the volume of its flow. The proportions of the total flows taken at high- and low-water stages are very different, and it thus happens that the average quality of the water taken for water-works purposes is different from the average quality of all the water flowing in the stream. Let us assume, for example, a stream having a watershed of such a size that in times of moderate floods water from the most distant points reaches the water-works intake in twenty-four hours. Let us assume further that rainfalls of sufficient intensity to cause floods and muddy water occur, on an average, once in ten days, and that the turbidity of the water at these times reaches 1.00, and that for the rest of the time the turbidity averages 0.10. Let us assume further that at times of storms the average flow of the stream is 100 units of volume, and for the nine days between storms the average flow is 10 units of volume. We shall then have in a ten days’ period, for one day, 100 volumes of water with a turbidity of 1.00, and nine days with 10 volumes each, or a total of 90 volumes of water with a turbidity of 0.10. The total discharge of the stream will then be 190 volumes, and the average turbidity 0.57. The turbidity of 0.57 represents the average turbidity all the water flowing in the stream, or, in other words, the turbidity which would be found in a lake if all the water for ten days should flow into it and become thoroughly mixed without other change.