CHARACTERISTICS OF RIVERS. g 



great, unless the major bed is confined by embankments. The two sides of the minor 

 bed are the banks, the right bank being always the one on the right hand of an observer 

 looking down stream and the left bank the one on his left hand. In many streams 

 they extend generally above flood-level, while in others the highest floods go over them. 

 In exceptional cases they are submerged by ordinary high-water stages. On nearly 

 all streams there are low banks at intervals, particularly at the mouths of the larger 

 affluents. 



The majority of rivers flow through alluvial lands, with varying strata of sand, 

 cky, and gravel as a foundation, underlaid by rock. Here and there the hills approach 

 on both sides, causing a narrowing of the bed, but the presence of hills close upon one 

 side is nearly always accompanied by bottom land upon the opposite side. In such 

 situations, if the soil is easily eroded, the constant action of running water will result 

 in a more or less continual changing of the banks and the bed. The amount of this 

 erosion will vary with the character of the material and the transporting power of the 

 water, which increases with the slope of the stream and with the mass of water in 

 motion. 



Basin. The territory drained by a river is called its basin. It therefore reaches 

 to the mountain-tops and includes the valleys of the tributaries as well as that of the 

 principal stream. Its highest point is the watershed which divides it from another 

 basin, while its lowest point is the bed just described. Its influence on the river is gov- 

 erned by the character of the strata of which it is formed. When these are permeable, 

 the rain sinks into the earth and reaches the bed gradually; when they are impermeable, 

 the water finds its way to the river rapidly, and the river will therefore fluctuate slowly 

 or rapidly as its basin is permeable or impermeable. 



Slope. The slope of a valley determines the velocity of the current of the stream 

 passing through it and is generally greater in the upper portion than in the lower. 

 When it is gentle very little erosion takes place, unless the soil is unusually unstable, 

 but, where the slope is great the water attacks the soil, and the river-bed takes a form 

 dependent upon the quantity of water and its velocity. As the river rises, its slope 

 and velocity will increase, and it is not possible for it in many cases to preserve a 

 fixed bed or banks. From this it follows that the profile cannot remain constant and 

 uniform for all stages and under varied conditions, nor can the cross-section. There 

 is no fixed relation known between slope and discharge, and the former varies with 

 the local conditions of the channel, which also constantly change, so that it may not be 

 the same for any length of time on a given length of river. 



Transportation of Sediment. The mountain-sides and valleys are slowly but surely 

 being carried to the sea by the rivers which penetrate them. In their upper portions, 

 where the slope is great and the velocity sufficient, the material transported is coarse 

 and heavy; as the descent of the valley proceeds this material becomes finer, until near 

 the mouths of rivers it is fine silt, capable of causing much trouble to navigation when 

 deposited in quiet water. This moving material, at whatever point in the river it is 



