In a country as lately elevated as New Zealand has been, none of its major streams has reached that uniform bed grade where all waterfalls, rapids, lakes, and like phenomena resulting from inequalities of profile have disappeared, hence waterfalls and rapids are common. They occur mainly where bands or zones of hard rock alternate abruptly with weak rock in the country into which the rivers are cutting their channels. The hard bands that have locally delayed downcutting may be beds of hard sandstone or limestone in the weak-rock mudstone country, more resistant layers in the hard rock highland, or sheets of hard volcanic rock interbedded with less resistant material. Stretches of rapids occur very commonly, but abrupt breaks in grade with spectacular vertical water drop are relatively rarer.
Some of the highest and best known of these latter occur in the glaciated high country of the South Island south-west where streams in little “hanging” valleys plunge over the high steep walls of the canyons shaped by the glacier ice that formerly filled them. Of such are the well-known Sutherland Falls in the Arthur Valley that drains into Milford Sound. From a small lake basin (a glacial cirque) perched on the ridge above the valley the water falls in three leaps, a total distance of 1,904 ft. Flow may diminish to a trickle in a spell of fine weather, but in times of heavy rain similar falls occur at many places along the valley sides. Well known, too, as an example of this type is the Bowen Fall in Milford Sound itself.
Typical of the falls formed where a downcutting stream finds a layer of hard volcanic rock are the Wairua Falls near Whangarei, and the Huka Falls just below the outlet of the Waikato River from Lake Taupo. The profile of stream channels, such as those which the Waikato and its tributaries have cut in the unconsolidated material of the central volcanic plateau, is very frequently broken where the streams find hard bands of the interbedded ignimbrite sheets. So small falls (Huka) and rapids (Aratiatia) are very common in this volcanic country.
The Wairua Fall, North Auckland, is more spectacular than these because the water drops vertically over the face of a nearly horizontal basaltic sheet embedded in country made of much weaker material.
Typical of the falls which have developed where streams cutting down into weak mudstone (papa) country find a hard sandstone band, is that in the Mangawhero River near Raetihi. Small falls and rapids of similar origin are common enough in the courses of the Wanganui, Mokau, and other rivers that drain the mudstone plateau of the North Island west.
On the two large maps (1 in. = 16 miles) accompanying the earliest comprehensive report on rivers and waterfalls as likely sources of hydroelectric power (Appendix D. 1A to the Journals of the House of Representatives, 1904), P. R. Hay lists no less than 78 waterfalls in the North Island and 163 in the South Island. Most of these represent steep descents (rapids) rather than vertical breaks (falls) in the stream profiles, but their number bears witness to the youth of the streams draining a mountainous terrain. G.J.