LAKES AND RIVERS. 



banks, that which is to the right in descending the river, being 

 called the right bank, and the other the left bank. 



32. Tributaries, or affluents, are the streams which flow into 

 a river on one side or other of its course. In the larger rivers 

 these tributaries themselves are often considerable rivers, and 

 receive along their course subordinate tributaries. 



By reason of the common tendency of water to find the lowest 

 level, rivers flow along the bottoms of valleys, and their winding 

 courses, often very complicated, are determined by the varying 

 direction of these valleys. Their tributaries run along the 

 bottoms of smaller valleys, intersecting that of the principal river 

 at various angles. 



33. The Valley, along the bottom of which a great river flows, 

 usually receives its name from that of the river, and is often of 

 vast extent; the declivities which form its sides sometimes 

 measuring hundreds, or even thousands, of miles. 



34. "Watershed is the name given to the declivities which 

 thus determine the .tributaries of a great river, and the whole 

 extent of the valley is sometimes called the basin or hydrographic 

 region of the river. 



3o. Delta, A great river, in approaching its mouth, often 

 diverges into different channels, forming angles with each other, 

 and thus discharges itself into the sea by two or more mouths. 

 These diverging branches are called a delta, from a fancied 

 resemblance, presented by the two extreme branches and the 

 line joining the two extreme mouths, to the Greek letter A, 

 delta. 



36. Estuaries. The mouths of rivers are often placed in inlets 

 of the sea, where the tide ebbs and flows, so that the waters of 

 the sea alternately enter the mouth of the river and retire from 

 it with the rise and fall of the tide, mixing with the water of the 

 river, and thereby producing a constant state of agitation in the 

 water of such an inlet. The name estuary has accordingly been 

 given to such sheets of water, from the Latin word cestus, signify- 

 ing the agitation of water such as that here described. 



37. Tirths. The name firth, also written frith, is some- 

 times given to estuaries ; this term, however, is more particularly 

 applied in Scotland. Thus the estuary of the river Forth, which 

 lies between Fifeshire and Edinburgh, is called the Firth of 

 Forth. 



The term firth or frith is generally assumed to be taken from 

 the Latin word f return t a strait or narrow neck of the sea. 

 Mr. A. K. Johnson, however, considers it to be derived from the 

 Scandinavian term fiord, pronounced jfiurth, which has the same 

 signification. 



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