Geological features of the reservoir basin, and of the dam site in particular, figure large in determining the feasibility of a project and the type of dam. In New Zealand the basic structure of the country has been greatly affected by orogenic forces not only in remote ages but also in renewed activity lasting into a much later period (Quaternary, Pleistocene) than was the case in most other land masses. This later uplifting is apparent both in the extensive faulting, folding, and fracturing so widely manifest in the geological structure, and in the present volcanic activity. Weathering agencies have been active in sculpturing the present deep relief, a legacy from which, of course, is the present dam sites.
Where old rock formations (severely worked by previous orogenesis) are within reach for dam foundations, they prove usually to be not only extensively affected by much cracking and shattering but often are also deeply deteriorated by penetration of weathering effects (e.g., the schist rock of Central Otago and the sandstones and mudstones of South Canterbury). Younger formations of marine origin, such as those of the Wanganui region, though more uniform through having escaped much of the former working, remain soft and weak. Volcanic and hydrothermal activity has added to the complexity of the problem by leaving large areas of the North Island covered successively with variable formations lacking density and strength and having high permeability. This applies in particular to the region traversed by the Waikato River, one of the country's more important sources of hydro-electric power.
A further effect of the aggressive weathering and late volcanic activity is that deposits of dam construction materials (gravels and sand for concrete and soils for embankments) tend to be lacking in both uniformity and extent where they are most wanted. Effects of earthquakes (an aftermath of the recent orogeny) present another factor to be taken into account. While dam sites traversed by active faults or situated close to them are avoided for obvious reasons, and those traversed by dormant or long inactive faults are treated circumspectly, it is not possible to proceed very far in the development of New Zealand's water resources without having to take into consideration faulting and other tectonic weaknesses. The provision of seismic resistance in dams is not novel, however, and has figured as normal practice in several countries. This factor, though, bears on the suitability of certain types of dam. Although the geological structure of New Zealand thus poses problems for dam building, none has proved unique; problems of a similar type have been encountered elsewhere, often in worse degree individually, and have been successfully surmounted. In matters pertaining to dams, New Zealand shares in the experience of other countries through membership of the International Commission on Large Dams, a section of the World Power Conference.