Waiapu River - Environmental Issues

Environmental Issues

In 1840, approximately 80% of the Waiapu River catchment area, covering 1,734 square kilometres (670 sq mi), was native forest, with a rich array of native flora and fauna. There was a small area to the east of the river covered in coastal forest and scrub due to partial clearance and burning. Between 1890 and 1930 there was large-scale clearing, felling and burning of native forests for pastoralism. Floods and heavy rainfall are common to the area, and this, combined with the development, resulted in widespread erosion and large amounts of sediment being deposited in the river. This has changed the landscape significantly.

Since the late 1960s, much work has been done to repair the area by planting exotic forests in eroding areas, and encouraging the return of native scrub. However, by 2002 the Waiapu catchment area had few natural habitats remaining. It was 26% exotic Pinus radiata forest, 37% pasture, 21% native forest, and about 12% kānuka and mānuka scrub. It was highly degraded and modified, and had extensive and serious erosion problems. About half of the pasture area could be considered erosion-prone and unsustainable. Many of the catchment's rivers were full of sediment, and classed as highly degraded. The river had one of the highest sediment yields in the world (20,520 t/km2/year in 2000), more than two and a half times that of the adjacent catchment area of the Waipaoa River. Anecdotal evidence suggests that this sediment may have adversely affected nearby coastal and marine environments.

Approximately one sixth of the annual sediment flow in all New Zealand river systems is in the Waiapu River, which continues to be one of the most sediment laden rivers in the world. The annual suspended sediment load is 36 million tonnes, and 90.47 m3 (3,195 cu ft) of sediment flows into the sea every second. The water quality of the river's tributaries is often much higher, as they are closer to the native vegetation cover of the Raukumara Ranges.

In lower areas, much of the eroded gravel from the catchment area settles on the Waiapu riverbed, making it rise rapidly. The riverbed rose 1 metre (3.3 ft) between 1986 and 2007, and a number of bridges over Waiapu's tributaries have had to be raised to accommodate their rising riverbeds.. As the riverbed rises, so does the river, which is causing extensive riverbank erosion. The banks eroded at a rate of 8 metres (26 ft) per year between 1988 and 1997. By the 2003 to 2008 period this rate had doubled, with 22 metres (72 ft) per year eroding in 2005 and 2006. This erosion threatens the town of Ruatoria, and groynes have been installed in an attempt to divert the river away from the town.