Adkin! — Readjustment of Drainage on the Tararuas. 189 



the northern side of " The Heights " basin, were actively extending their 

 sources headward into the outer slopes of its rim. One of these streams 

 finally cut back completely through the rim, and tapped a portion of " The 

 Heights " stream. Invigorated by this success, it still further extended 

 its course witliin the confines of the basin, ultimately capturing half of the 

 drainage-lines therein. Proof of this act of piracy and the consequent 

 reversal of drainage is furnished by the fact that the stream now flows in a 

 direction contrary to the slope of the area it drains, as shown in fig. 1, (Note 

 altitudes — corrected aneroid readings — of the surface of the old alluvial flat.) 



Simultaneously, or approximately so, a further change took place, this 

 time in the south-west part of " The Heights " basin. There, one of the 

 aggrading streams so far raised its bed that it at last overtopped a low 

 place in the main Arapaepae divide, and a spill-over course resulted, by 

 which its waters were diverted westward into the catchment area of the 

 Koputaroa Stream. This type of stream-diversion has been described by 

 Gilbert as "' diversion by alluviation." The alternative possibility, that the 

 spill-over course at " The Heights," and also that on the Poruriri Ridge 

 (described in section 5), were cau.sed through capture by the headwater 

 erosion of streams rising outside the basins of the diverted streams, is 

 rejected on the following grounds: (1.) Both the Arapaepae and Poruriri 

 Ridges have fairly even, unnotched crest-lines, and headwater erosion of 

 the streams draining their western slopes does not now, or at any pre- 

 vious period, appear to have caused, or even tended to cause, diversion 

 by piracy. (2 ) The original surfaces of the alluvial flats in the basins 

 of the two diverted streams referred to above still overtop the former low 

 parts of the Arapaepae and Poruriri ridge-crests respectively. In each 

 case the notch cut by the spill-over course can be differentiated from the 

 low part of the ridge-crest. 



In this manner the former coalescent drainage of " The Heights " basin 

 became divided into three distinct parts, each of which possesses either 

 inherited or newly-acquired topographic characteristics. The remaining 

 undiverted portion of the original drainage bears, all the signs of advanced 

 age, and still pursues a meandering course on the surface of the alluvial 

 flat before flowing south-east to join the Makahika River. The streams 

 captured by the pirate stream flow in narrow youthful trenches at a depth 

 of from 15 ft. to 100 ft. below the old alluvial surface, and then plunge 

 into the deep and narrow gorge that forms the breach in the northern rim 

 of the basin, and join the Mangaore. In one place part of this entrenched 

 drainage lies only about 7 chains from the old meandering stream, and 

 further captures are thus imminent. The stream diverted by alluviation 

 has also entrenched itself to a slight extent into the alluvial flat, and then 

 cascades down the steep western face of the Arapaepae Ridge to augment 

 the waters of the Koputaroa. The changes described bear the marks of 

 extreme youth, and further adjustments must eventuate before even a 

 moderate state of stability is attained. 



5. Changes of Drainage on the Poruriri Ridge. 

 The Poruriri Ridge is another section of the outermost western foothills, 

 lying between the vents of the Mangaore and Tokomaru Rivers ; and linearly 

 it is the northward continuation of the Arapaepae Ridge. The diversion 

 of drainage which took place on the Poruriri Ridge was similar to but less 

 complex than that at '' The Heights," since in the former locaUty the act 

 of piracy was not committed. In the former locality the change was due 

 to diversion by alluviation causing a spill-over course (now known as the 



