Problem of the Anomalous Lower Course of the Waianakarua River, North-east Otago
[Read before Geol. Sect., Wellington Branch, July 10, 1952; received by Editor. November 22. 1952.]
The lower course of the Waianakarua River cuts through high ground in a dnect course to the sea, abandoning an easier earlir south-east channel. Processes by which a drainage channel may breach a topographic barriei me weighed, and a combination of intercision and capture by headward erosion is given preference on the observed evidence. A map showing geographic and relief features and sketches of four postulated stages illustrate the hypothesis advanced.
The locality map (Fig. 1) shows the geographic layout of the area and includes the 100ft, 200ft and 300ft contour lines; trig stations and spot heights supply additional details of the relief. The encircled letters, A, B-C, and D, refer to the three principal surfaces (see corresponding sketches) successively developed during the evolution of the present topography and the drainage
adjustment, and correspond to the four stages described and then listed in the summary.
The crossing of a high topographic barrier by a river or other natural drainage channel is possible by any one of several recognized processes; it may be antecedent to the barrier, superposed upon it, or of spill-over origin following temporary ponding. None of these appears to be applicable to the anomalous lower course of the Waianakarua. Other means by which a barrier may be breached and drainage diverted through it are capture by headward erosion and intercision. It is suggested that it was by a combination of these two processes that the Waianakarua attained its present direct course to the sea.
Fig. 2A shows an early stage in the development of its present lower course, with the Waianakarua flowing over a fan-like deposit that filled a still earlier more southward-trending channel (McKay, 1887), and in the final phase of aggradation also spread over higher ground to the north. If no gravel or other alluvial cover is found within this northern range of the river at this stage, the accordant surface extending in that direction will be one of lateral planation of bedrock. Lookout Bluff Hill, composed of resistant igneous rock, survived planation and became isolated and its lower slopes buried, by the aggrading surface, thus becoming an inlier similar to Burnt Hill, the Gorge Hills, and others, on the present Canterbury Plain. A relatively low sea-level apparently accompanied this stage.
In Fig. 2B, the Waianakarua River has ceased aggradation, and after an interval of equilibrium has commenced to degrade its course. The term of rejuvenation apparently was brief, and was followed by one of still-stand lasting long enough for a broad flat-bottomed channel trending south-east to be developed. A meandering course, with migration of meanders and much lateral cutting of the steep sides of the trench, marked this phase. Moreover, a tendency at this stage was for the river to impinge strongly on its left bank at the major bend below its debouchure from its upper, hillenclosed valley. By persistent lateral corvasion the bend became a broad and deep left-bank re-entrant (Fig. 2C)—a contributory factor in the abrupt change in course that came to pass later.
In the meantime minor streams nearer the coast, draining from the hilly country immediately to the north and also, to a lesser extent, those from Lookout Bluff Hill, joined up on the north-eastern part of the fan-form surface developed in the stage shown in Fig. 2A, and flowed to the sea between Lookout Bluff Hill and the northern hills. Probably, though not necessarily an essential factor in the course of events suggested, a relative rise in sea-level had taken place and the outer margin of the older fan surface had become cliffed during coastal recession. The minor stream-system therefore soon developed youthful, steep-gradient gullies which rapidly extended headward into the earlier fan-form surface, especially in the direction of the deep re-entrant cut by lateral corrasion by the Waianakarua at the pronounced bend below its upper debouchure.
Partly by intercision and partly by converging headward erosion of the contiguous drainage units—the Waianakarua and the minor stream-system respectively—a break-through took place (a heavy flood in the Waianakartia probably being the final factor), and the present direct course to the sea
through the high ground that contrasts so strikingly with the deserted lower former south-east channel, became established. This direct shorter course to the sea at once led to a re-grading of the river bed to a still lower level, leaving the deserted south-east channel high and dry m its present form (Fig. 2D).
Summary and Captions of Fig. 2.
Stage A.—Widespread alluviation around an isolated hill of resistant rock. with recurrent courses of the river, simultaneously or successively, on either side of it. River on present 100-feet surface (terrace). Relatively low sea level.
Stage B.—Downcutting and formation of flat-bottomed trench trending south-east. River on present 40-feet surface (terrace). Minor stream-system initiated in earlier eastern course of river and incising youthful gullies. Cliffrecession under marine abrasion at relatively higher sea level.
Stage C.—Widening of floodplain in trench to form deep re-entrant at point) where river impinges on left-bank cliff at major bend in course. Minor stream-system enlarging and deepening its gullies and extending headward.
Stage D.—Lateral corrasion at major bend, as in Stage C, continued until intercision took place, the river breaking through via the minor gully-system to form the present direct course to sea. Development of present floodplain by regrading, about 30–40 feet below earlier south-east channel.
Mckay, A., 1887. N.Z. Geol. Surv. Rep. Geol. Explor. 1886–87 No. 18: 234, map at p. 1.