Reviews—Doyne’s Rivers in New Zealand. 269 
On THE Rivers AND PLains OF CANTERBURY, NEW ZEALAND. 
By W. T. Doyne, Esq. 
{2 Reporting to the Secretary for Public Works, Christchurch, 
Canterbury, New Zealand, in June 1864, on the selection of a 
site for a bridge over the Rakaia, on the prevention of further en- 
croachment of the Waimakariri on Kaiapoi Island, and on turning a 
creek of the Rangitata, Mr. W. T. Doyne* states that these three 
rivers, taking their rise high up in the mountains, and chiefly fed by 
snow and glaciers, are necessarily very fluctuating, higher in summer 
than in winter, and apparently more affected by hot north-westerly 
winds than by rain. Though generally fordable, they form, in their 
‘freshes,’ torrents of great power, spreading over shingle-beds from 
one to two miles wide, and at places cutting new channels to the 
depth of 20 feet. In the mountains, for 40 or 50 miles, each river 
cuts out and washes down great quantities of boulders, gravel, and 
sand, carrying it out through the gorges, along the river-courses, to 
the sea; a portion being deposited on the way, according to the 
diminishing force of the stream, either from less fall or increased 
width. The material in the river-beds is all siliceous, greatly water- 
worn, and in every variety of size and form, from impalpable dust 
to boulders measuring two cubic feet. As this shingle accumulates 
in the bed of the stream, the water is thrown off against the low 
banks of similar material; and these being quickly undermined, the 
river widens out until it has no power to carry forward any but the 
lightest sand and mud. Thus barriers rise several miles in length, 
raising the bed of the river above the banks, and making it overflow ; 
until before long it cuts out a new course, deepening and widening 
it, until the old course is completely left and restored to the plains; 
the new channel becoming the true river. In time this course goes 
through the same changes, and it again is filled up. As the plains 
diminish in fall towards the sea, this tendency on the part of the 
rivers to fill up their beds increases; and in this way all these rivers 
are now so gorged, that they cannot move the shingle forward beyond 
a point about fifteen miles from the sea. At this point each river 
must now overflow its banks, and find a vent in new channels, unless 
prevented from so doing by considerable engineering works. The 
rivers differ, of course, in their volume of water and in the fall of 
their beds; but Mr. Doyne arrives at this conclusion, that, when the 
inclination of the river-bed, composed of shingle and sand (as in the 
Rakaia and Rangitata), is less than 24 feet in a mile, the space over 
which these rivers can change their courses must be confined to 
within half a mile in order to keep the course open. Where the 
river runs through silt and clay, a much less fall is required, as with 
the Waimakariri near its mouth. He believes that the Canterbury 
Plains, extending from NE. to SW. for 100 miles, with a width of 
from 30 to 40 miles from the sea-coast to the foot of the mountains, — 
may be looked upon as the delta of these great rivers, having their 
* In the ‘ Press,’ Christchurch, N. Z., for July 1864. 
