CHARACTERISTICS OF RIVERS. 13 



It is usually not a difficult matter to cut through a shoal and secure the depth 

 required for navigation; but in so doing new troubles are liable to arise because the pool 

 level above may be lowered and new obstructions uncovered. If a cut be made, its 

 limits in width and depth should therefore be rigidly fixed to those actually required, 

 in order to maintain the level above. The reduction of section of flow will also result 

 in increased current velocity which will cause some trouble to ascending navigation 

 and endanger that descending. For this reason the new channel should be as straight 

 as practicable. 



Bars. Bars are formed of material transported and deposited by the water so as to 

 form a ridge, and are usually on the crossings, that is, somewhere in a line which a boat 

 will take in passing from a pool in the bind at one bank to a pool in the bend along 

 the opposite bank. On alluvial rivers of steep slope they are not, like shoals, always at 

 the same place ; however, they usually form at about the same point each year. They are 

 the inevitable result of the general travel of alluvium cut from the banks or from the 

 hills near the sources of the -stream, and of the accidents of slope common to all river 

 basins, and, while they may be removed from one point, if the material is deposited below 

 high-water line, it is probable that it will be again found in a new bar farther down the 

 river, although it may possibly be in water so deep that navigation will not be disturbed 

 by it. If some of the bars are reduced as fast as formed, it may result in a less number 

 but in greater dimensions of those remaining. 



It not unfrequently occurs that bars are formed from external causes, such as the 

 construction of a pier, breakwater, or other structure which may deflect the course of 

 the current in such a manner as to erode the bank or the bed of the stream. In such 

 cases the removal of the bar, in all probability, will not be followed by its renewal if 

 the erosion is stopped by proper protection works. But with bars formed by the trans- 

 portation and deposit of material on its journey from the mountains toward the sea, 

 no removal by portable appliances or other means can prevent their reproduction, and 

 the best that can be hoped for from such sources is a temporary relief. 



Changes in Level and Floods. The low-water and high-water levels of a river can 

 usually be easily ascertained, but they are both subject to fluctuation, and the con- 

 struction of works of improvement may change the conditions to such an extent as to 

 seriously affect previously established levels. On most navigable rivers the lowering 

 of the low- water level would cause a great inconvenience to navigation, and this is true 

 also of the filling up of the river-bed. As a general thing the raising of the high-water 

 level will not seriously affect navigation, but it may cause loss and suffering along the 

 adjacent lands, and is, therefore, to be avoided. As a country is cleared up and cul- 

 tivated, and the forests removed, the water carries vast quantities of material into the 

 stream, because much of the rainfall reaches the river rapidly under the changed 

 conditions. The result is that floods are more frequent and more violent, and the banks 

 of the river and its tributaries .are cut away and carried into the stream. This action 

 is aggravated by the employment of splash-dams and the removal of obstructions in 



