Tane (1971) 17:173-179 173

RESULTS OF THE WAITANGITAONA RIVER COURSE CHANGE, MARCH 1967

by G. Rennison*

During a flood in March 1967, the Waitangitaona River, South Westland, overtopped and breached its left-hand stopbank, establishing itself on a new course towards the sea. This action has resulted in an interesting chain of events occurring along the new course, with pasture, forest, lake and lagoon communit• ies all suffering a degree of modification. The White Heron and Spoonbill colonies near Okarito lie within the sphere of influence of the Waitangitaona River, and while it is uncertain at the moment that the course change has had any direct effect upon them, the colonies' situation would appear to be more precarious than prior to 1967.

GEOLOGICAL BACKGROUND Movement along the active zone of the Alpine Fault has produced a vast amount of fragmented schist rock, overthrust across the fault to the west. This phenomenon is encountered in several places in South Westland, but in the headwaters of the Waitangitaona catchment it has produced a grave problem, as a massive slip in this fractured schist is pouring debris into Gaunt Creek, and from there into the Waitangitaona River. The Waitangitaona is unable to carry away this debris faster than it arrives, and so is aggrading, or building up its bed, downstream of the Gaunt Creek junction. This problem has been recognized for some time, and various schemes have been considered to effect some measure of control over the river. During the 1920s, when land development for dairying was in progress, flood overflows were reported more frequently than before, and construction of the left-hand stopbank was commenced in 1933. This was a substantial structure, nearly a mile long and between six and fifteen feet in height. However, subsequent erosion and aggradation reduced its usefulness, and by the 1967 river bed was approxi• mately ten feet above the level of the ground on the other side of the stopbank. The flood of March 1967 breached the stopbank and the whole flow of the river was transferred to a new course, across an ancient alluvial fan to the southwest of the breach. Most of the river's flow then proceeded to Lake Wahapo, down the outlet river (Wahapo River) into the Okarito Lagoon and thus to the sea. Some of the flow was directed from the fan into Graham Creek, and back to the old Waitangitaona bed.

EFFECT ON COMMUNITIES (1) Communities along the New Course (Wahapo to Okarito Lagoon) First to suffer was an area of 1,830 acres of farmland, shared amongst six

* Westland National Park 174 properties. The most heavily damaged farm had houses and farm buildings wrecked by the river, and shingle was spread .thickly over the grassland. The area is a depressing sight from the highway, with blackberry and coarse sedges spreading over the least damaged sections. Small branches of the river meander along shingly courses which are at greater elevation than the highway in one place. As the Waitangitaona River proceeded across the old alluvial fan to the southwest, the heavier fractions of its load were quickly deposited. Thus when the river entered a stand of Kahikatea (Podocarpus dacrydioides) at the northern end of Lake Wahapo, a large silt deposit was laid down as the streams pushed through the forest. In places the deposit is over 5 feet thick, and has completely buried the ground and low understory vegetation. The Kahikatea trees are now dying over much of the silted area, a most unhappy fate for one of the few such stands in Westland National Park. Proposals have been made to log the stand and salvage the timber, but in places the degradation of timber due to fungal attack following the trees' death has gone too far for salvage to be possible. Immediately downstream of the Kahikatea stand is Lake Wahapo. Prior to the 1967 floods this was a typical South Westland glacially-originated lake, with dark, peaty waters giving beautiful reflections of the forest around the lake's shores. The lake is a product of the waning of local glaciers at the end of the last glaciation - moraine walls enclose the lake to the north, south and west. To the east a large, gently sloping alluvial fan with forest, swamp and Pakihi bog forms the shore-line. The catchment was small, with numerous tiny streams discharging from the fan and moraine walls into the lake. A small outlet river (Zala's Creek), breaches the moraine in the west and, together with the outlet stream from , (Okarito River) Hows into the Okarito Lagoon. The construction of State Highway No. 6 along the southern shore ensured the Lake's popularity as a stopping point for visitors. The lake's catchment area now effectively includes the whole of the Waitangitaona River mountain catchment of 27 square miles. It is obvious that the alteration of the tiny lake's ecosystem by this dominant new factor must be great. In January 1969 several Westland National Park lakes, including Lakes Mapourika and Wahapo were subjects of a study by Dr Stout, of the University of Canterbury, who obtained information on temperature, light penetration, and plankton numbers. In a broad sense we can regard Lake Mapourika as representing the pre-1967 condition of Lake Wahapo, and directly compare the two lakes. Three points of note come from the January 1969 work. First, there is a much greater amount of suspended sediment (silts) in Lake Wahapo, and increased turbidity. Light penetration is much reduced. Second, there is almost uniform water temperature from the surface to the bottom in Lake Wahapo, compared with the temperature stratification found in Lake Mapourika, which has the typical condition of an upper layer of warmer water (epilimnion), separated from the lower mass of colder water (hypolimnion) by a sharp transition zone (thermocline). The large volume of water flowing through the small basin of Wahapo must preclude, by its mixing effect, the formation of a temperature gradient. Third, and obviously linked with the previous two, there is a smaller 175

FIG. 1 EFFECTS OF WAITANGITAONA RIVER COURSE CHANGE (1) Gaunt Creek. Schist material transported into Waitangitaona River. (2) Western or left-hand stopbank breached by March 1967 floods. (3) Pastureland overwhelmed by alluvium and now carrying braided channels of new river course. (4) Kahikatea stand, silted and dying at the head of Lake Wahapo. (5) Lake Wahapo. (6) Okarito River now carries most of the flow of the Waitangitaona River down to Okarito Lagoon and to the sea. (7) Okarito Lagoon. (8) White Heron and Spoonbill Colonies on Waitangiroto River. (9) Pre-1967 course of Waitangitaona River. (10) Shore Lagoon. 176 amount of plankton in Lake Wahapo. There can be no doubt that the pre-1967 ecosystem of Lake Wahapo can no longer exist. An ecological disaster has certainly befallen the lake. Scenically, the lake is a somewhat diminished asset, with its milky-brown water, fluctuating water levels and dying Kahikatea forest. The lake is part of Westland National Park, and the Park has perhaps suffered a loss in its modific• ation. However, a National Park exists for another, different purpose from that of scenery preservation - it exists to document natural changes in the physical features which it contains, and the breaching of the lake by the Waitangitaona River is a natural phenomenon - no work of man has greatly affected it, except the stop-bank, which merely postponed the inevitable. (Even the noxious animal extremists cannot get a bite in here - the Gaunt Creek schist was on the move before browsing became a problem). When the water of the Waitangitaona River leaves the lake, it links with the outflow from Lake Mapourika to form the Okarito River, flowing into the Okarito Lagoon. The very much larger flows down the river since 1967 have brought about some bank erosion, but little obvious damage has occurred. The effect on the Okarito Lagoon, as far as can be discerned from conversations with locals, is two-fold. Effect on the lagoon as a community appears to be nil, as the huge area of the lagoon acts as a buffer against the increased river flow. Two effects may be operating. First, increased water flow may tend to keep the lagoon mouth open more effectively than before. Secondly, once the sea has thrown a really good bar across the mouth, the increased flow then speeds up the flooding of low-lying land around the township area. (Hypothetically this may also speed up the bulldozing of the bar which is occasionally necessary, so that the net effect may be considered to be a speeding up of the bar-flood-dozer cycle!)

(2) Communities along the Old Course (old channels, shore lagoon and White Heron Colony on the Waitangiroto River)

After the March 1967 flood, about 800 acres of former riverbed were abandoned by the Waitangitaona River. This does not mean that all water flow has ceased, as ground water still wells up in places along the old channels, and some flood waters spill over from the Lake Wahapo fan towards the old riverbed, so that flow is maintained down the lower reaches of the river into the tidal lagoon at the mouth. Inextricably tied up with the fate of the Waitangitaona River is the well- being of a small, sluggish neighbouring river, the Waitangiroto, which is of great importance as it is the only site in where the White Heron and Spoonbill Heron nest. The Waitangiroto River obtains its flow from groundwater, emerging from a large tract of swamp land, to flow slowly west into a common lagoon with the Waitangitaona River, and until recently, the River. Although evidence is as yet inconclusive and much work remains to be done, it seems that the Waitangiroto is subject to interference, and the communi• ties to environmental modification, from two sources - the headwaters swamp 177 and the coastal lagoon. Dealing with the headwaters swamp first - it is certain that the Waitangitaona River, while it flowed to the sea along its old course, must have regularly spilled floodwaters over the low catchment boundary into the Waitangiroto headwaters. Although not proven, it is likely that this regular flooding helped maintain a stable environment along the lower reaches of the river, where the Heron Colony is located. Since 1967 no floodwaters have crossed from the Waitangitaona to the Waitangiroto; none will as long as the Wahapo course is followed. This may have a deleterious effect on the Waitangiroto River communities. A further threat may present itself. Now that the Waitangitaona River' is "safely out of the way" the agricultural development of swampland in the upper part of the Waitangiroto is feasible. Whether this will eventuate we do not know. An interesting sequence of events has developed at the coast, where the rivers form tidal lagoons before flowing across the beach into the sea, and for this information I am indebted to Dr Peter Wardle, as well as various local sources. However, before looking at this sequence of events some mention of the habits of lagoons and lagoon sandbars is necessary so that the results of such events are more easily seen. When the rivers in the area are low or when storms create heavy surf along the beaches a sandbar may be built across the river mouth and the river water begins to bank up. Eventually the bar will be breached and a rapid lowering of the lagoon's water level takes place. Once the bar is destroyed it may be weeks or months before it is replaced and tidal effect is felt in the lagoon and tributary rivers. Tidal effect along any particular tributary is greatest when the breached sandbar is directly opposite the tributary mouth.

THE SEQUENCE OF EVENTS In 1964, whitebaiters bulldozed a cut through the beach ridge opposite the i mouth of the Waitangitaona River, in order to give the whitebait access to the streams and rivers flowing into the lagoon. In August 1965 the Whataroa, Waitangitaona and Waitangiroto Rivers emptied into the same lagoon system, with a single channel to the sea. By August 1967 the Whataroa had re-formed its own channel to the sea. The lagoon opening to the sea used by the Waitangitaona and Waitangiroto Rivers gradually eroded its way south until in August 1967 it was opposite the mouth of the Waitangiroto. Maximum effect of tidal surge and wave action in the lower reaches saw increased bank erosion. In March 1967 the Waitangitaona River changed course into Lake Wahapo, thus removing the possibility of floodwaters reaching the common lagoon system), and reducing the threat to any sandbar thrown across the lagoon entrance. (The shortly after this found a new exit from the lagoon system.) In October 1968 a heavy gale drove high seas across the beach and lagoon, resulting in salt spray killing vegetation, and consequently causing accelerated erosion of the lagoon and lower Waitangiroto river banks. A new sandbar has now formed across the Waitangiroto mouth and the lagoon opening is once again opposite the mouth of the Waitangitaona, thus reducing tidal wear and tear on the former. 178

It is obvious that when you are dealing with shifting river mouths, aggrading rivers, unstable sandbars, plus the unpredictable influence of white• baiters with bulldozers you are going to have your work cut out to guess what is going to happen next, and where. However, a few facts are clear. (1) The Waitangitaona River is not likely to return to its former course without massive human assistance which currently is not considered. (2) The diversion into Lake Wahapo of the floods tha* flow down the Waitangitaona must have a significant effect c both the Waitangiroto swamp and the lagoon system. (3) The closing of the bar opposite the Waitangiroto mouth and the location of the lagoon opening opposite the Waitangitaona mouth have created a more favourable situation in the lower reaches of the Waitangiroto River.

In spite of all the trials and tribulations caused by the rivers, the 1969 breeding season for the Herons was an excellent one, showing that if there is an ecological disaster hanging over them, they have not noticed it yet!

CONCLUSION Various communities of widely different type have been affected by the sudden and permanent course change of the Waitangitaona River. The amount of effect varies widely - the pastureland, Kahikatea forest and lake communities have been most heavily modified, the Okarito River and lagoon areas seem little disturbed. The "fringe communities" of the Waitangiroto River, their environ• ment in some way dependent on the old Waitangitaona River, are presumably reacting, but in ways as yet unclear. This, perhaps, is the most disturbing feature of the whole business, but at the same time the most interesting. As a pointer to the future of the area, the Westland Catchment Board in its report makes it clear that the present course of the Waitangitaona River into Okarito Lagoon is the most satisfactory from the point of river control, as they can see great alluvium storage capacity on the old fan and in lake Wahapo. From the National Park point of view, we should not be greatly disturbed by what is after all a purely natural advance in the landform development of the area, even though it causes temporary destruction of some scenic values and one of our few Kahikatea stands. It is obvious that more investigations, especially with regard to the White Heron Colony, are urgently needed. The possibility of changes in the Waitangi• roto River's behaviour should act as a spur to obtain more and better information on the Heron and its unique New Zealand nesting environment.

ACKNOWLEDGEM ENTS Thanks are due to N.Z. Geological Survey, Westland Catchment Board, Drs. Vida Stout and Peter Wardle for allowing access to their unpublished reports. I 179 acknowledge the assistance of both the Westland National Park Board and National Parks Authority, and state that any opinions offered are my own an not necessarily reflect Board or Authority thinking.