I 4 THE IMPROVEMENT OF RIVERS. 



the smaller streams for the purpose of flooding out the timber therein. This constant 

 inflow of material under the influence of great velocities and steep slopes has the result 

 of gradually filling up the pools and reducing the depths for navigation. In one 

 instance which has come under our observation surveys made of a river of steep slope 

 in 1875 showed a series of deep pools separated by shoals. After that time the country 

 was stripped of much of its timber, farms were opened up, splash-dams were built in 

 many of the tributaries for bringing out logs, the river itself was improved by clearing 

 its bed and confining its channels, and to-day such a thing as a deep pool is not known. 

 They have practically all filled up, so that where, twenty years since, the water was 20 

 feet deep at low stage, it is now not that many inches. The pools have not only filled 

 up but the low-water level and river-bed have been raised in the lower portion of the 

 stream, the tendency being to form a bed with a regular slope, and in many cases these 

 grade-lines have obliterated the smaller shoals entirely. This new bed is in many 

 places entirely above the old low-water level. Thus in the lower part of the stream 

 a gauge was set with its zero 9 inches below low-water mark. Ten years later the low- 

 water mark was 2 feet above the zero, with really a less discharge in the river than at 

 the time the gauge was fixed, and in the same period all the shoals and pools within 10 

 or 15 miles of the gauge had disappeared and regular slopes had been established. 

 This action was not due to the construction of any improvement works in the 

 vicinity. 



In the study of a project for open navigation it is of importance to allow for 

 the variation between the approximate low water which is known and the extreme low 

 water which may occur under certain conditions, but the position of which is not known. 

 The approximate stage is generally called conventional low water, and is a plane of com- 

 parison a little arbitrary, but almost coinciding with the lowest water, and consequently 

 giving a basis on which to work. The next stage above conventional low water is ordi- 

 nary low water, that is, the depth ordinarily reached by the river at periods when there 

 is barely sufficient water for navigation without a reduction of loads. At this stage 

 boats proceed with more care and less speed. 



The highest navigable stage is also one of importance, particularly in fixing the 

 level of bridge floors. It is seldom practicable to raise the coping of works of navigation 

 above the level of the highest navigable water in this country, although this practice 

 obtains in parts of Europe where the flood variation is not so great. The level at 

 which navigation ceases necessarily varies, and can be increased if boats are built 

 stronger, and if more powerful means of propulsion are employed. When a river 

 reaches a certain stage, however, navigation becomes very difficult, and even dangerous, 

 on account of the increased velocity of the current as well as from the uncertainty of 

 the directions of its forces. Ordinarily navigation ceases when the water overflows the 

 banks, but it will vary with circumstances and at different points on a stream 

 according as the banks are high or low, or the river comparatively straight or crooked. 



It is also of importance to know the height of the greatest flood. Like conven- 



