The contrast between the two classes of rivers is well brought out in a study of the Lahn and the Wieseck, by Kisskalt (1906); and the table below, compiled from his data, gives an excellent idea of the total numbers of bacteria and their seasonal fluctuations in a stream of fair quality (the Lahn) and a highly polluted one (the Wieseck).In the former case the bacterial numbers are highest when rain brings surface pollution; in the latter, when the sewage constantly present is least diluted. MONTHLY VARIATIONS OF BACTERIA IN A NORMAL AND POLLUTED STREAM Kisskalt, 1906 Date. Bacteria per c.c. Date.