24 



AQUEOUS AGENCIES. 



vated ; this will reflect the current obliquely across to the other side, 

 which will become similarly excavated. Thus the current is reflected 

 from side to side, increasing the excavations. In 

 the mean time, while erosion is progressing on the 

 outer side of the curves, because the current is 

 swiftest there, deposit is taking place on the inner 

 side, because there the current is slowest : thus, 

 while the outer curve extends by erosion, the in- 

 ner curve extends, pari passu, by deposit (Fig. 

 15), and the winding continues to increase, until, 

 under favorable circumstances, contiguous curves 

 on the same side run into each other, as at a b, 

 and the curve c on the other side is thrown out 

 and silted up. Thus are formed the crescentic 

 lakes or lagoons (I I) so common in the swamps of 

 great rivers. They are abundant in the swamps 

 of all the Gulf rivers, especially the Mississippi. 

 They are old beds of the river, thrown out and 

 silted up in the manner indicated above. 



5. — Flood-Plain Deposits. 



All great rivers annually flood portions of 

 level land near their mouths, and cover them with 

 sedimentary deposits. The whole area thus 

 flooded is called the flood-plain. These flood- 

 plains are very extensive, and the deposits very 

 large, in the case of rivers rising in lofty mount- 

 ains and flowing in the lower portion of their 

 course through extensive tracts of flat country. 

 In the lofty mountains the current runs Avith 

 great velocity, and gathers abundant sediment ; on reaching the flat 

 country the velocity is checked, the river overflows, and the sediment 

 is deposited. The flood-plain of the Mississippi Eiver is 30,000 square 

 miles. The flood-plain of the Nile is the whole land of Egypt. 



The flood-plain of a river may be divided into two parts, viz., the 

 river-swamp and the delta. The river-swamp is that part which was 

 originally land-surface ; the delta, that part which has been reclaimed 

 from the sea or lake by the river. We will take up these in succession. 

 River-Swamp. — We have already seen that, with every recurrence 

 of the rainy season or of the melting of snows, the flooding and the 

 deposition of sediment are repeated. Thus the river- swamp deposit 

 increases in thickness, and the level of the whole flood-plain rises con- 

 tinually. Fig. 16 is an ideal section showing the manner in w 7 hich the 

 flood-plain is successively built up : a a a is the supposed original con- 



Fig. 15. — Three Successive 

 Stages of a Meandering 

 River. 



