Lake waters are generally almost entirely free from sediment. At Chicago the city water drawn from Lake Michigan has slightly more than 1 part in 100,000 of suspended matters, as determined by Professor Long in 1888-9, and by Professor Palmer in 1896. The suspended matter in this case is probably due to the nearness of the intake to the mouth of the Chicago River, and to mud brought up from the bottom in times of storms. The lake-water further away from the shore would probably give much lower results.
Turning now to waters having considerable turbidities, at Pittsburg the average suspended matters in the Allegheny River water, as shown by the weekly or semi-weekly analyses of the Filtration Commission during 1897-8, were 4 parts in 100,000. During a large part of the time the suspended matters were so small that it was not deemed worth while to determine them, and the results are returned as zero. This is not quite correct, and a recomputation of the amount of suspended matters, based on the observed amounts, and the amounts calculated from the turbidities when they were very low, leads to an average of a little less than 5 parts in 100,000, which is probably more accurate than the direct average. The average turbidity on the platinum-wire scale was 0.16.
At Cincinnati the suspended matters are about 23 parts in 100,000, and at Louisville about 35 parts, both of these figures being from Mr. Fuller’s reports. In all these cases the enormous and rapid fluctuations in the turbidity of the water is a most striking feature of the results.
Observations on the Mississippi River above the Ohio have been made by Professor Long in 1888-9, and by Professor Palmer in 1896. These results are not as full and systematic as could be desired, but indicate averages of 20 to 30 parts in 100,000 at the different points. Professor William Ripley Nichols, in his work on water-supply, states the amount of suspended matter in the water of the Mississippi, probably referring to the lower river, as 66.66 parts.
Investigations of Professor Long and Professor Palmer for numerous interior Illinois streams extending over considerable periods give average results ranging from 1 to 8 parts in 100,000. The very much lower results for the interior streams as compared with the Mississippi and Ohio rivers may be due to the relative sizes and lengths of the streams, or in part to other causes.
Regarding muddy European rivers there are but few data. The Maas, used for the water-supply of Rotterdam, is reported by Professor Nichols as having from 1.40 to 47.61 and averaging 10 parts of suspended matters in 100,000. More recent information is to the effect that the raw water has at most 30 parts of suspended matters, and that that quantity is very seldom reached.
At Bremen the Weser often becomes quite turbid. The turbidity of the water is noted every day by taking the depth at which a black line on a white surface can be seen. Assuming that this procedure is equivalent to the platinum-wire procedure, the depths at which the wire can be seen, namely, from 15 to 600 millimeters, correspond to turbidities of from 0.04 to 1.70, a result not very different from the conditions at Pittsburg.
At Hamburg and Altona the water is generally tolerably clear, but at times of flood the Elbe becomes very turbid, and the amount of mud deposited in the sedimentation-basins is considerable. At Dresden, several hundred miles up the river, I have repeatedly seen the river-water extremely turbid with clayey matter, the color of the clay varying from day to day, corresponding to the color of the earth from which it had been washed.
At Budapest, where filters were used temporarily, the Danube water was excessively muddy with clayey material. At first very high rates of filtration were employed and the results were not satisfactory. Afterward the rate of filtration was limited to 1.07 million gallons per acre daily, and good results were secured. There was no preliminary sedimentation. Professor Nichols reports the average suspended matters in the Danube at 32.68 parts in 100,000, but does not state at what place.
Many of the French and German rivers drain prairie country not different in its general aspect from the Mississippi basin, and the soil is probably in many places similar. There is no reason to suppose that the turbidities of these streams in general are materially different from those of corresponding streams in the United States, although it is true that, other things being equal, the average turbidity of water taken for water-works purposes will increase with the size of the stream; and it may be that some American streams, especially the Ohio, Missouri, and Mississippi rivers, are of larger size than European streams, and consequently that the turbidity of the water taken from them for water-works purposes may be greater.