TRANSPORTATION BY RIVERS 251 



Velocity re- Velocity re- 



Size of pebbles quired to move quired to start 



after stirring up motion 



Hazelnut size 0.923 m. per sec. 1.35 m. per sec. 



Walnut size 1.062 m. per sec. 1.39 m. per sec. 



Pigeon egg size 1.123 m. per sec. 1.45 m. per sec. 



The transportation of the coarser detritus in streams is chiefly 

 by rolling along the bottom, more rarely by pushing along. The 

 entire river bed may be in motion, forming a waste stream saturated 

 with water, which in the case of the Rhine at Ragaz and the Birsig 

 at Basel has been found to possess a depth of more than three 

 meters (Pestalozzi-53 :vi) and of four meters in the Danube at 

 Vienna. This phenomenon is, however, ordinarily restricted to 

 moinitain streams in flood. 



The sands of the river bottoms generally assume the arrange- 

 ment of a series of low banks, alternating in position on opposite 

 sides of the stream. Opposite each low bank is a deep channel with 

 steep sides, harboring the main current of the river. These banks, 

 which in general have a triangular outline, their bases against the 

 river banks, slowly wander downstream, through removal of the 

 sand on the upstream side, its passage across the bank and deposi- 

 tion against the downstream side. On the middle Rhine such banks 

 move at the rate of 200 to 400 meters per year downstream, this 

 being increased to three times that amount in years of high water. 

 In seven years a bank may thus reach the former position of the 

 one next downstream on the same side. ( Grebenau, cited by Penck- 

 51 •.286.) In the regulated reaches of the Danube at Vienna, the 

 sand banks have wandered in seven years from 700 to 1,000 meters 

 downstream. {V^nck-^i: 286.) In the Loire the sand banks mi- 

 grate at the rate of 1.72 to 3.61 meters per day in summer and 2.37 

 to 18.65 "1- P^'' ^^y "^ winter, according to the varying angle of 

 fall of the water, which is from 0.28 permille to 0.45 permille. This 

 also shows the great difference between the transport in summer 

 and winter, the latter being the season of floods and high water gen- 

 erally. In all cases the motion of the waste matter is much slower 

 than that of the water, and the amount of water passing a given 

 point may be a thousand times the amount of sediment carried 

 past that point in the same period. Thus the Rhine above Germers- 

 heim carries nearly 7 cu. cm. of waste for every cubic meter of wa- 

 ter, while the Danube at Vienna carries on the average 13 cu. cm. 

 in every cubic meter of water. That sands and pebbles can be trans- 

 ported in the course of time to great distances by river currents 



