THE pW ZEALAND 
AL OF SCIENCE 
AND 
HNOLOGY. 
Vol. IV. Wellington, March, 1921. 
No. 1. 
MANGAHAO HYDRO-ELECTRIC SCHEME. 
The Structure of Mangahao No. i Gorge, and its Bearing 
on the Construction of the Proposed Dam. 
fit 
By G. Leslie Adkin. 
Those who have made themselves conversant with the details of the 
Mangahao hydro-electric scheme, and who are familiar with the topo¬ 
graphy of the area in which the proposed works are to be situated, will be 
aware that the Mangahao Biver furnishes the principal supply for the high- 
level reservoirs, and also that the construction of an effective dam across 
that river is one of the principal features of the scheme. The delivery of 
the available “ head ” of water to the power-house, which is to be located 
near the inner margin of the Horowdienua lowland, involves the construc¬ 
tion of two reservoirs and a surge-chamber—all of high and practically 
similar elevation—and the piercing of two lengthy tunnels ; but the whole 
scheme hinges on the resistance of the dam to be located at the spot known 
as No. 1 Gorge to the sudden and powerful floodings of the Mangahao 
River. 
The Mangahao is one of the larger tributaries of the Manawatu River. 
It rises in the principal hydrographic centre of the Tararua Range, and 
pursues a north-easterly course for a distance of about forty miles before 
]oining its trunk river at the upper end of the Manawatu Gorge. The upper, 
intermontane course of the Mangahao occupies a valley which is for the 
most part longitudinal, but in part transverse, to the longitudinal ridges 
of the Tararua Range. Erratic courses of this character are common to, 
and are the most striking feature of, all the Tararua river-valleys : the 
transverse reaches are invariably narrow, rock-bound gorges, while the 
longitudinal stretches are in a more advanced stage of development, and are 
for this reason more widely opened, having flat shingly bottoms. Short 
lengths of rock-bound gorge may be found here and there in the longitudinal 
parts of these river-valleys, their origin in such circumstances being due to a 
comparatively recent lateral shift of the river-course at the places where 
they occur. Mangahao No. 1 Gorge belongs to this class. 
At No. 1 Gorge the Mangahao Rivej? flows through a steep-sided, rock- 
bound trench some 8 chains in length, 15 yards to 20 yards in width, and 
about 50 ft. in depth. The formation is strong greywacke intersected by 
wide-spaced joints, and as a dam-site the spot is at first sight ideal. On 
its eastern side the gorge is surmounted by a fragment of high-level terrace 
1—Science. 
