SECT. 3] 



ESTUARIES, DELTAS, SHELF, SLOPE 



633 



the year, but in summer catastrophic floods rush from the south, where the 

 break-up of the ice occurs distinctly earher than in the delta: the mean dis- 

 charge of the Lena River in June is said to be as large as 2,275,000 ft^/sec; in 

 other words it nearly reaches the absolute maximum discharge reported from 

 the Congo River. This huge mass of water, carrying ice-rafts and trees, is often 

 not able to find its way in the delta through the existing distributaries, since 

 these are still frozen, so that it commonly overtops the banks and creates new 

 channels every year. The resulting network is very confused. Concerning the 

 grain size of the sediments, Carsola (1954) gives information for the Mackenzie 

 River, which appears to carry to the open sea only fine silts and clays, either in 

 suspension or through ice-rafts (Fig. 22). It would be interesting to know the 



Fig. 12. North Mahavavy delta, Madagascar, showing a braided- channel pattern. (Photo. 

 A. Guilcher.) 



part played by wave action on the delta front: it may be supposed that it 

 would be of very small extent, because the sea is frozen at the time when waves 

 could overcome the river forces. 



Another type includes intertropical deltas in which disastrous floods occur 

 owing to the great rains which fall over large areas during hurricanes or 

 ty|)hoons. The North Mahavavy delta in the north-west of Madagascar belongs 

 to this type (Fig. 13). Each big hurricane results in a new course. The channels 

 are braided and basins exist as distinct units. The sediments in channels are of 

 two types: coarse sand deposited during floods and fine sand, silt and some clay 

 deposited during low discharge. Micaceous silts of low density are found on 

 levees. The clay content is a maximum in the basins, although coarse quartzose 

 elements are present. On the seaward side, between meandering tidal creeks 

 extensive mangroves grow on mud which includes an important coarse fraction 



