14 DIMINISHED FLOW OF EOOK RIVER. 



exception of a slight return to normal conditions in the fourth period, 

 there has been a decrease in precipitation from 1885 to the present 

 time. 



The foregoing facts, though they do not determine to what extent 

 the diminished precipitation has affected the now of the river — since 

 corresponding figures for temperature, evaporation, and wind velocity 

 are wanting — nevertheless suggest one of the obvious reasons for the 

 change. In attempting to weigh the importance of this factor it 

 should be remembered that the greatest fall from the average of 30.4 

 inches for any three consecutive years was 3.3 inches. This does not 

 appear to be a very serious loss, considering the conservative tendency 

 of the geology of this region. The table shows, moreover, that the 

 months from April to September, inclusive, when water is most needed 

 to sustain the flow, have decidedly the heaviest precipitation. 



Admitting, therefore, that a diminished rainfall may have influenced 

 to some extent the volume of the river flow, the disturbance in the 

 regulation of that flow must be ascribed to other causes. Where the 

 average annual rainfall is just sufficient for the ordinary purposes of 

 agriculture and the industries, as in the present instance, the distribu- 

 tion of the supply becomes important. In order that a sustained flow 

 of water may be secured, the spring rains and the melting snow should 

 be economized for later distribution. The combined supply from these 

 two sources is considerable. Where it is allowed to penetrate into the 

 earth and thus find its way gradualty to the streams, it constitutes a 

 safeguard against the urgent needs of the warmer season, during which 

 heavy intermittent showers produce a more rapid run-off, not infre- 

 quently followed bjr periods of drought and intense evaporation. 



This disturbance in the regulation of the water flow of the Rock 

 River must be ascribed partly to the artificial drainage of cultivated 

 areas, but chiefly to the changes that have taken place in the soil cover 

 since the time of settlement. 



ARTIFICIAL DRAINAGE. 



The custom of laying tiles and other drains and of cutting ditches to 

 improve the condition of the fields prevails throughout this region, 

 and has resulted in a more rapid delivery of the rain water into the 

 streams. Many of the swamps and sloughs that formerly helped to 

 feed the smaller creeks and tributaries have likewise been drained to 

 bring their rich soil under cultivation. 



The case of the Horicon Marsh deserves to be specially noted as an 

 example of drainage. The history of the changes that have taken 

 place on this marsh also points to the possibility of an improvement 

 in the water supply by a system of storage. The tract is situated at 

 the northern end of the river, and is, in fact, its first source (PI. II). 

 As early as 1846 the marsh was converted into a lake for water-power 



