Chain-saws not the first threat to heron colony

Press, Volume CXVI, Issue 34098, 10 March 1976, Page 9

Chain-saws not the first threat to heron colony

(From cor TV ellington reporter) The future of the white heron nesting grounds on the Waitangiroto River, eight miles north of the Okarito Lagoon. is threatened by the encroachment of farms and by geographic changes to the coastline as well as by any treat the milling of the Okarito State forest may pose.

For more than two years the Soil Conservation' and Rivers Control Council has been working towards a compromise which would satisfy the requirements of both the local farmers and the white herons, but the predicated changes to the coastline are considered beyond the power of man to stop.

Major floods in South Westland in 1967 created changes in the established channels of the two adjacent rivers — the Waitangiroto (where the white herons nest) and the Waitangitaona. In March, the Waitangitaona burst its left bank upstream, to establish a new course through Lake Wahapo and down the Okarito River into the lagoon. In August, the Waitangiroto was found to have by-passed the lagoon it has shared with the Waitangitaona, and cut dirct access to the sea.

1967 threat

Both changes were recognised at the time as constituting a threat to the continued existence of New Zealand’s only known nesting area of the white heron. Investigations of the changes were begun immediately, to “provide a framework within which future work could be done to the mutual satisfaction of all parties.” In his published report in 1974 Mr W. B. Morrissey, of the Water and Soil Division of the Ministry of Works, said that both, changes “gave rise to the possibility of a confrontation between environmental and local farming interests.” The issue was summed up as long ago as 1945, when the then director of the Canterbury Museum, Dr R. A. (Now Sir Robert) Falla, said: “Nothing but a substantial block of swamp forest (well worth it for its own sake) will ensure permanent and complete sanctuary for the white herons of Westland.” The course of the Waitangiroto is held under four 1 different classes of land tenure:-

The Waitangiroto Fauna and Flora Reserve.

Crown land, some of which is farmed.

Lands and Survey’ Department land, out on "renewable

pease to local farmers, with ;the right of purchase. i Freehold land, farmed to a greater or lesser extent. Private swamp The Waitangiroto is only, lat most, nine miles long, land its major source is the j Waitangiroto swamp. This ■ swamp is situated on land [that is entirely freehold or ‘held under perpetual lease, and only the lower section of the river is on reserve or Crown land. The Lands and Survey leasehold land is only 250 yards from the river.

After Mr Morrissey had reported in 1974, the Soil Conservation and Rivers Control Council agreed to establish a buffer zone of about 1620 hectares (4110 acres) around the colony, and said that this proposal would require the purchase of 1200 hectares (3000 acres) of freehold and renewable lease land.

! The Westland Catchment Board was asked to design any drainage works on land adjacent to the reserved area in such a way that the outlets would, as far as possible, discharge away from the catchments of both the Waitangiroto and Waitangitaona Rivers. The Council then asked the Catchment Board, last September, to prepare a catchment scheme for the W aitangiroto River separating the land suitable for drainage from the land which should be reserved to protect the quality of the water in the Waitangiroto.” This would mean that the run-off from the drained land would floss’ into the old bed of the Waitangitaona, which had not been envisaged initially, and the Catchment Board is expected to complete the survey soon. The change in emphasis away from the Waitangitaona can be explained by the amount of seepage and underground drainage dossil the old bed of the river, noss - the river has settled into ; ts new course. In effect, the Council svants the major portions of the Crown land and freehold land incorporated into the present Waitangiroto reserve, and probably parts of the Lands and Survey renewable lease land as svell. Mr Morrissey described the Waitangitaona River as being “the central issue in the controversy,” and work has: been continuing for years to establish exactly svhat, if any, bearing water levels in the Waitangitaona have on the Waitangiroto. It svas a “typical West Coast rain river,” he said, draining the coastal ranges and very unlike its larger neighbour, the Whataroa River, which drained the perennial snow and icefields of the main divide.

In 1918, a massive slip had occurred in a tributary

of the Waitangitaona, just above the State Highway 6 bridge. The resulting debris had raised the level of the riverbed downstream, and over the years the stopbanks had had to be raised repeatedly to maintain the river in its course. Local farming interests, while not happy about the immediate damage caused when it burst its left bank in 1967 and began flowing through Lake Wahapo, had been satisfied with the change because it freed extensive downstream farmlands, and enabled the banks to be “more fully developed and used.’’

Control scheme The Westland Catchment Board has now prepared a scheme to control the course of the Waitangitaona into Lake Wahapo, and to prevent any possible diversion back into its old course or, even worse, into the Waitangiroto, which in places is less than half a mile from the old Waitangitaona bed. The Waitangiroto drains the left-bank terrace of the Waitangitaona, which has been formed by flood silts. It is at present a deep, placid, stream carrying, during severe floods, a large volume of floodwater from the old bed of the Waitangitaona. This occasional flooding, coupled with the comparative absence of any bed load, is believed to account for its unusual depth and steep banks, which are ideal for the white heron colony. Tidal ranges in 'its estuary are usually less than one metre but, depending on the position of the outlet to the sea, the tide can range up to four kilometres upstream from its lagoon.

Sea threat During exceptionally low flows or violent westerly storms, when at high tide river movement is stilled, it is possible for one or all sea outlets of the three rivers to be closed by the extension of the existing beach ridges. The enclosed rivers then flood the lagoons behind the beach ridge until they find the weakest point or points to make one or more new outlets.

The positions of these new outlets may be maintained until floodwaters from a particular river force a direct passage to the sea. The continued absence of floods in one river may allow the other rivers to dictate the positions of the openings. The three outlets are caught between two moraine ridges — Abut Head and Waitaki Bluff — and are thus restricted to three miles of coastline. In August, 1965, three rivers shared a common outlet well south of the usual outlet opposite the Whataroa River, and by August, 1967,

it had shifted further south' to directly opposite the Waitangiroto. By 1970, the Whataroa had its own mouth, but in the intervening period the increased tidal activity in the lower! reaches of the Waitangiroto' had caused bank slipping, a process easily accelerated because of the unusually steep banks and deep bed. Mr Morrissey said the connection between these events was not clear. It seemed probable that they were not connected, the sea erosion being a cyclic occurrence which was affecting all West Coast beaches.

But a local non-cyclical cause for the erosion of the beach ridges was the progressive erosion of the coast ends of the two moraine walls. This process was very evident south of the mouth of the Waitangiroto, where a normally stable area of several hectares, together with huts used by whitebait fishermen, had been destroyed.

Inland erosion Erosion was also occurring along the vegetationcovered coastline inland of the three rivers’ lagoons, with high-tide waves constantly overtopping the beach ridges at many points. “Unless there is a marked reduction in natural erosion processes, the present beach ridges appear doomed, and with them considerable amounts of the coastline, including the lower reaches of the Waitangiroto,” Mr Morrissey said. “The threat to the white heron sanctuary is obvious, but little can be done in this direction. “The best that can be hoped is for an orderly retreat upstream of the heronry in the face of increased exposure to westerly gales,” he said. The Soil Conservation and! Rivers Control Council hasj been keeping an eye on this! situation, but considers any scheme to preserve the; coastal ridges is financially; impossible. The council has made informal approaches to the landowners and occupiers adjacent to the Waitangiroto, and will make formal approaches to them to buy the land needed once an over-all plan for the area has been completed. The Council’s task has been made harder by the longstanding nature of most of the farming units. The quality of the water in the Waitangiroto, which could be subject to enrichment by the use of fertilisers within its catchment and so have a weed problem, has been studied regularly since 1970 by a technical committee comprising representatives of the Westland Catchment Board, the Rivers Control Council and the Nature Conservation Council.