A river runs through it – World Rivers Day

Department of Conservation —  27/09/2020

If you’ve just fallen into a reverie thinking about the film that launched Brad Pitt’s career (you’re welcome), let’s harness that swoon and transfer it to a love of rivers/awa for World Rivers Day. Specifically, let’s talk about the importance of a whole catchment approach to river restoration.

A tributary feeding into a wetland system that leads to the Hōteo River.
📷: Susan Stoddart

From the mountains to the sea, rivers run through many landscapes with corresponding uses including native forest, farming, horticulture, forestry and urban areas. The various activities associated with these uses affect the awa, and therefore their wider catchment. To understand this connection better, we’re taking a deep dive into two rivers and their catchments in the Tāmaki Makaurau/Auckland region: the Hōteo River which flows into the Kaipara Harbour, and the Mahurangi River which flows into the Hauraki Gulf Marine Park/Ko te Pātaka kai o Tīkapa Moana.   The two rivers form part of the Department of Conservation’s (DOC) Ngā Awa river restoration programme.  They were chosen because their headwaters are in a natural condition, important ecosystems are present, local communities are interested in, or already involved in, restoration work, and the catchment has factors that could be improved or fixed. A lot of work has already occurred to improve the rivers’ health by iwi/hapū, councils, government, Crown entities, landowners, landcare and community groups. DOC’s involvement is an intention to add value to the collaborative efforts through dedicated river rangers who will help create a collective vision, and ultimately a successful outcome.

But first, why is a catchment, and why are our rivers so special?

A catchment is an area where water is collected by the natural landscape. Imagine cupping your hands and collecting water in them; your hands have become a catchment. The outside edge of a catchment is the highest point, where gravity causes all rain and run-off to run downhill where it naturally collects in rivers, creeks, wetlands, lakes or oceans.

Waiwhiu Stream, a tributary in the Dome Valley in the Hōteo Catchment.
📷: Susan Stoddart

New Zealand has more than 70 major river systems that make up an incredible 425,000 km of waterways. New Zealand rivers have some unusual characteristics: many are short and steep, are highly variable in flow, and have very large floods in proportion to their catchment area, due to our country’s latitude, climate and mountainous relief.

A clean flowing river supports the life of the catchment, providing moisture for soil, nourishment for species and people, a place of peace and recreation, and, for many, spiritual solace. Restoring the ecological integrity of our river catchments will contribute to the long-term resilience of freshwater systems and the native species they support. It also means we can swim safely in our beautiful awa.

What threats do our rivers face?
Sediment in the lower reaches of the Hōteo River, 1 km from the mouth at Mangakura.
📷: Susan Stoddart

Too many. Sediment, nutrients, faecal contaminants, dumped rubbish and wastewater enter our waterways and pollute the fresh water. Also, freshwater weeds invade, impacting the quality and flow of rivers and exotic fish compete with native species for food and habitat. Sedimentation (eroded soil entering waterways) is the most pressing problem facing rivers and their wider catchments. Generally, this is caused when rainfall destabilises soil from cleared land or steep slopes and large-scale land developments (i.e. motorways, roads and subdivisions). While a small amount is accepted as ‘normal’, excess sediment causes damage by smothering habitats, impacting the ability for fish to forage, leading to both the reduction in and destruction of aquatic life.

Hōteo River -long, secretive and cherished
Kayaking on the Hōteo River
📷: Titine

At 28km long, the Hōteo is the longest river in the Auckland region. It begins as a trickle from Te Arai Scenic Reserve on the east coast (near Mangawhai, north of Auckland), combining with other tributaries until it becomes a river running through pastoral land, steep native forest, gully systems and freshwater wetlands. It supports a number of ‘at risk-declining’ native fish including giant bully, torrent fish, inanga and longfin eels. After it travels the breadth of the isthmus, it ends its journey at the Kaipara Harbour on the west coast, New Zealand’s largest estuarine ecosystem. The Kaipara contains rare coastal saline ecosystems such as sand dunes, seagrass, freshwater and estuarine wetland ecosystems, which support diverse intertidal fauna.

Whitebait/ inanga.
📷: Stephen Moore

Before European settlers arrived, mana whenua thrived on the Hōteo River and Kaipara Harbour’s plentiful food sources. When settlers came, they logged trees for their precious timber, after which the land and a mosaic of wetlands and swamps (which help clean water), were drained and converted for farming. More recently, many of the steep hills have been planted in pine forest. Some native bush remains (9%), but without trees and plants to anchor the soil in most of the catchment, sediment is deposited in the river. The Hōteo River is now the second-largest contributor to the sediment build-up in the Kaipara harbour, which stresses both freshwater and marine environments. DOC River Ranger, Sue Stoddart, explains, “The Hoteo would once have been a vast mosaic of wetlands. I have a vision of them being restored, protected and allowed to fulfil their nurturing role once again.”

Mahurangi River – an ancient highway and biological hotspot
Mahurangi as it flows out to the sea from Warkworth Village
📷: Susan Stoddart

The Mahurangi River, also known as Awa Waihē, begins as two branches, fed from sources that begin at The Dome and Moir Hill. They converge and flow through the town of Warkworth (Puhinui), before dispersing into an estuary and then Mahurangi Harbour. The harbour then opens into the Hauraki Gulf. Early in the catchment, remnant pockets of native forest remain which shelter rare plants including native mistletoe and orchid. Long tailed bats/pekapeka-tou-roa, kauri snails, lizards and the New Zealand pipit/pīhoihoi are also found here. Further down, the estuarine area supports a number of wader, shore, and wetland birds including the banded rail/moho pererū and pāteke. The river’s endpoint, the Hauraki Gulf Marine Park, is a biological hotspot that supports a multitude of species including whales and dolphins, seabirds, sharks, fish and a myriad of marine habitats.

Banded rail/ moho pererū
📷: Andris Apse

Mahurangi River has a long history of use by Māori and settlers as a major transport route. The awa was an abundant source of kai and freshwater for mana whenua and was protected by defensive pā. After European settlers arrived, the trees were felled, and land was converted to pastoral farmland. Many industrial sites were established, using scows and steamers to export the goods. Today, the Mahurangi catchment is a mixture of pasture and productive rural land combined with scattered urban and lifestyle subdivisions, as well as growing development around Warkworth town. Historical deforestation and land use intensification have led to high sedimentation in the Mahurangi river, estuary and harbour, and ultimately, the Hauraki Gulf Marine Park.

What can you to to help improve the health of our rivers?
Jose Watson, Kim Bryan-Walker and Celine Stokowski undertaking riparian planting.
📷: Richard Rossiter

Whole catchment restoration can seem a daunting task, but collectively we can do a lot to clean up our rivers. One of the best things you can do is participate in community group planting days to plant the areas along rivers (riparian planting). This helps to prevent erosion, filter and cool the water, and provide food for aquatic insects and therefore, fish.

To find your local community group, check out Nature Space.

Love it, Restore it, Protect it