Ohinemuri Regional History Journal 8, October 1967
By (late) P.G. MORGAN, M.A.
It is a remarkable fact that the Ohinemuri River has some of its sources within a mile of the east coast, and yet has a westerly course for about fifteen miles (as the crow flies) before it joins the Thames or Waihou River, which flows northward into the Firth of Thames. Moreover, the range of hills it pierces in the lower part of its course is much higher than the coastal range that excludes its headwaters from the open sea. The wide plain-like upper valley is another remarkable feature.
The Ohinemuri River, as a map name, begins north-east of Waihi, where the Mataura and Homunga streams unite. After flowing southward for two miles from this point it is joined from the south-east by the Waione. It then turns sharply west, and making its way round the south side of Black Hill, is joined from the south by the Waimata and Mangakiri streams, from the north by the Mangatoetoe and the Waitete. At Waikino, four miles west of Waihi, its size is increased by a fairly large tributary, the Waitekauri, coming from the north. The Tieri [Taieri - E] and several small streams enter it near Owharoa, and it then passes into the Karangahake Gorge. Here it is joined by a tributary of almost equal size, the Waitawheta, which drains a large area to the south in Aroha Survey District. At Mackaytown the Ohinemuri emerges from the gorge. Its valley expands, and the stream, flowing through widening flats, enters the Hauraki Plain, and at Paeroa joins the Waihou River.
As soon as the various tributaries of the upper Ohinemuri reach the Waihi Plain they meander over it in narrow, slightly entrenched channels. The small streams from the east and south usually flow through swamps for a short distance. Below the swamps low terraces or beaches cut in soft rhyolitic tuff (wilsonite) may border the streams. The Waimata drops over a small fall not far above its junction with the Ohinemuri, and other small tributaries have a little rapid where they enter the main stream. The Ohinemuri itself near Waihi flows at grade in a rather narrow channel with banks 20 ft. or 30 ft. high, and as far down as Owharoa, there may be a narrow bench on one or both sides, beyond which, in several places there are distinct but low terraces, cut in wilsonite, etc. At Waikino the bench on the south bank widens to form a small river-flat, on which the Victoria Battery was built. Stream-gravels and finer debris to some extent veneer the terraces or benches.
From Waihi to Owharoa the fall of the river, though gentle, is greater than the westerly slope of the Waihi Plain, and consequently at Waikino the river is well below the general level of the neighbouring part of the plain. The Waitekauri joins at grade, but the Tieri (Owharoa Falls) and other small streams descend from the plain to the Ohinemuri by falls and cascades. Below Owharoa the grade of the Ohinemuri, as it enters the Karangahake Gorge, steepens. Emerging from this gorge, the stream again flows at grade. For the last part of its course, near Paeroa, it is tidal, and (was once) navigable by boats.
Several of the early writers on the geology of the Hauraki Goldfield; thought that the Ohinemuri River at one time flowed eastward to the Bay of Plenty. Bell and Fraser, adopting this view, suggest that the direction of its drainage was reversed owing to the ponding of its present upper valley by extrusions of rhyolite near the eastern coast-line, presumably where the road to Waihi Beach crosses the coastal range. If, however, the old outlet of the Ohinemuri had been so blocked, its waters failing an outlet elsewhere, would almost certainly have found their way into the Waiau Gorge, through which the road to Tauranga now passes. Moreover, Henderson has shown that the Ohinemuri River had a westerly course in its early days. Elsewhere he gives the impression that at a still earlier time it drained eastward, but the name Ohinemuri cannot well be applied to such a stream.
The Ohinemuri, sinceit came into existence, some time during the Pliocene period, and its chief tributary the Waitawheta, have cut gorges through the north flank of Karangahake Hill as it was gradually elevated in late geological time. The wide ancient valley of the Ohinemuri canstillbe plainly seen at 700 ft. or less above the stream-level at Karangahake. Evidently the land just here was gradually uplifted 600 ft. or 700 ft., probably during Late Pleistocene time. The Ohinemuri and the Waitawheta, being streams of considerable size, were able to deepen their channels as the uplift progressed, and thus to continue flowing westward, as formerly. They are therefore antecedent streams in this part of their course.
Between the Mangatoetoe and Waitete streams, north-west of Martha Hill there is a well-marked, rather high, south-facing terrace, forming part of what is known as the Kensington or Riri Estate or freehold. This also is in the main a rock-bench. South of it, near the western ends of Rata and Albert streets, are two small terrace steps facing south. On the west side of the Waihi golf-links, (Mr. V. Hollis' property) (now Waihi College), and as far south as Toomey Street, there is a very distinct terrace-drop, facing the Mangatoetoe Stream. Small but well-formed terraces occur in the Waitekauri Valley above the Grace Darling Creek, and there is also a distinct terrace in the lower part of the Grace Darling Valley. Well-formed terraces, apparently all gravel, may be seen in the valley of Hikutaia Stream. These terraces are directly connected with recent land-elevation, but the terraces and rock-benches of the upper Ohinemuri are probably results of late stages in the cutting-down of the Karangahake Gorge, an operation which is still in progress.
Rock-benches are more or less distinctly seen in the western part of the Karangahake Gorge at heights reaching 250 ft, or more. Hector observed coarse water-worn drift in this locality at a height of 200 ft. above the river.