466 RIVER ESTUARIES. 



rapids and other obstacles, which render it therefore of 

 little account as a navigable stream. The plains track, 

 on the contrary, is generally through a comparatively 

 level country where the river flows in a wide, deep, 

 and tranquil current, and this portion of its channel in 

 a great river is very often navigable, sometimes for 

 hundreds and occasionally even for thousands of miles. 

 The river Amazon, for example, together with its 

 tributaries, it is computed, affords free navigation over 

 not less than 30,000 miles, all, or almost all of which 

 lies within Brazilian territory. * 



Then the estuary or tidal portion of a river, which 

 is of such vast importance as a port of entry for ocean- 

 going ships, is subject to a great variety of conditions, 

 which differ very much in character on every river. 

 In the first place there are of course the different falls 

 of the river beds, and next there is the question of the 

 rise and fall of the tides, which in some places is very 

 great and in others comparatively small. Thus the 

 Mersey, with an extreme tidal rise of 30 feet at its 

 mouth, is only tidal for 46 miles ; whilst the Seine, with 

 a rise of 22 feet, is tidal for 91 miles; and the Scheldt, 

 with a rise of only 13^ feet, is tidal up to Ghent, 105 

 miles, f 



When a river enters the sea abruptly, the silt which 

 is carried out by the current is sure to be driven 

 back by the waves and to be deposited in obstructing 

 barriers near its mouth, forming dangerous bars, on 

 which the surf during storms beats with terrific 

 violence; the entrance to such rivers in bad weather 



* Encyd. Brit., gth edit., Vol. iv., p. 221 (Article "Brazil"). 

 t Encycl. Brit., Qth edit., Vol. xx., p. 576 (Article "River Engin- 

 eering "). 



