﻿154. Transactions. — Miscellaneous. 



sive snow-fields occasioning enormous floods. From tlie rapid fall in the river 

 beds, many of them having a greater inclination than thirty-three feet to a 

 mile in their' upper parts, vast quantities of drift shingle and silt are brovxght 

 from the higher levels and deposited in the lower, where the velocity of the 

 stream is diminished by the lessening inclination of the bed ; the fine gi-avels 

 and sand are borne onward to the sea, and form the bars and shoals which 

 exist at the mouths of all our rivers. 



The larger gravels are thrown down as the velocity of the stream diminishes, 

 and rapidly fill tip the lower portions of the river bed until it is raised above 

 the level of the adjoining land, when the stream, during some fiood, overflows 

 its banks, inundating all the low ground adjoining, and makes a new channel 

 in the lower ground, which in its turn will be rapidly filled up, and the stream 

 overflows again into the lowest ground in the neighbourhood. To this action 

 is due the formation of most of the alluvial flats bordering the lower poi"tions 

 of the river courses. This subject has been veiy carefully investigated in 

 connection with the formation of the Canterbury plains, by Dr. Haast and 

 Mr. Doyne, C.E., whose reports and maps are most instructive. The process 

 of successive elevations of the river bed is much more rapid in an open 

 coimtry such as Canterbury, where there is nothing to prevent the destruction 

 of the river banks, than it is in a wooded country, such as the Nelson province, 

 but the process is exactly the same in both cases. In a forest-covered country, 

 such as the greater portion of the IsTelson province is, the elevation of the 

 river beds is necessarily a slow process. The forest clothing the mountain 

 sides checks the sudden rush of water down hill during rain, besides preventing 

 it from cutting water-channels in the surface, thus preventing a supply of 

 detritus to the river in the first instance ; and also the banks of all streams 

 and rivers in a state of nature, before disturbance by the hand of man are, 

 thickly covered with scrub and ferns, which, hanging down into the water, 

 constitute a most efiective protection against the destructive action of the 

 rivers. The natural vegetation which covers the surface of a wooded country 

 may be truly said to form the best protection to its surface, and the difierence 

 in the manner in which the water of precipitation drains off" forest and open 

 land is very striking, and well worthy of attention. Wlien heavy rain falls 

 on forest land, before it begins to flow on the surface it has to saturate all the 

 humid and decaying vegetable matter which lies at the foot of the trees ; the 

 surface of the ground is also so covered with a network of roots that the water 

 can only form a series of pools, which overflow from one to another as the 

 rain continues, and a large body of water is thus retained upon the ground, 

 which di'ains ofi" slowly through the moss and roots. The small water-courses 

 also get filled with trees, masses of twigs and moss, which materially assist in 

 checking the velocity of the streams, and prevent abrasion of the surface — but 



