510 Transactions. 



its two remaining extremities, which may still be seen — one near the 

 northern end of the Koputaroa Road, and the other a little north of th(^ 

 north-east corner of Levin Borough. 



North of this ridge the drainage was then the same as now, but on the 

 southern side a considerable change has occurred. The Upper Koputaroa 

 .Stream — i.e., the portion south of the dividing-ridge, took its rise in the 

 southern extremity of the Arapaepae Range, flowed parallel to it as far 

 as the present position of Queen Street, and then, bending north-west and 

 west, flow^ed into the Ohau River near the point where it then entered the 

 sea. The Upper Koputaroa was divided from the Ohau River by another 

 low sandstone ridge extending from near the source to the junction of the 

 former with the latter. The " Kaiwha " ridge (such being the name by 

 which it was known to the old-time Maoris) varied from a quarter to 

 half a mile in wddth, and its summit was about 25 ft. above the beds of the 

 Upper Koputaroa and Ohau Rivers, though later it was in some parts 

 considerably lowered — apparently by pluvial erosion. 



While deepening and widening its valley the Upper Koj)utaroa removed' 

 part of the ridge which divided it from the lower part of its present valley,, 

 and, deserting its connection with the Ohau, formed a continuous stream,, 

 which poured its waters into the Manawatu River. A base-level of vertical 

 erosion having been established, lateral erosion became important, so that 

 the Koputaroa attacked and removed the upper portion of the " Kaiwha " 

 ridge, thereby gaining access to the old " north-west " channel of the Ohau 

 River, which was by that time vacated, and therefore dry. The surface- 

 slope of the Ohau fan preventing an overflow of the Koputaroa to the 

 south-west or west, deflected it toward the north-west and caused it to 

 hvig the southern side of the " Kaiwha " ridge, and by lateral erosion to 

 cut back its bounding scarp. The sandstone of the ridge was, however, 

 not so entirely removed as to exj)ose the underlying fan. 



After attaining the base-level of erosion as mentioned above, the 

 Koputaroa began to fill its valley with alluvium derived from the Arapaepae 

 Range. This deposition of material was probably the result of a similar 

 process on the part of the Manawatu River— the one keeping pace with the 

 aggradation of the other. Being in a state of stable equilibrium, the Kopu- 

 taroa (when the pivot of its motion was in the breach of the " Kaiwha " 

 ridge) by repeated oscillations from one side of its course to the other 

 flowed and deposited material, sometimes on the south side, and sometimes 

 in its valley proper, on the northern side of the " Kaiwha " ridge. This 

 process was continued until the alluvium had reached its present thickness 

 and structure. 



During its recent outfiowings along the line of Queeji Street to the centre 

 «tf Levin a second bifurcation of the Koputaroa Stream occurred, this 

 time near the Horowhenua Butter-factory. At this point the stream by 

 building up its bed was able to cross a low portio)i of the " Kaiwha " ridge, 

 and after an interval, and again by aggradation, this time of its newly- 

 acquired channel, it entered the shallow^ valley of a small stream which 

 formerly flowed westward into Lake Horowhenua. Taking possession of 

 this small valley, the Koputaroa deposited material in it for the remainder 

 of the time of its outflow in this directio)i, thrustiiig forward its alluvium 

 as far as the intersection of Queen Street and Tirotiro Road. Both the 

 first and second branches of the westward outflow of the Koputaroa Stream 

 reached and entered the then more extensive Horowhenua Lake. 



