Water catchments

A water catchment is an area of land and the water that collects and moves through it. A catchment is often bordered by hills or mountains. The water enters via precipitation (rain or snow) and then moves across the surface or subsurface (as groundwater) until it drains into a stream or river. The water flows into the lowest parts of the landscape and eventually leaves the area via a single point or outlet. The outlet may be another stream or river, wetland, lake, estuary or the ocean.

Catchments can be small or large

Catchments vary in size. They can be quite small – for example, a gully system or stream that runs through an urban neighbourhood or from a steep hilly slope. These smaller catchments – known as subcatchments – are usually tributaries that flow into larger streams or rivers. If you look at a hilly or mountainous area on a topographic map, you can see where streams begin and how they join up to create larger subcatchments.

The Waikato River catchment

The Waikato River catchment is an example of a large catchment area. At 14,456 km², it is the second-largest catchment area in the country. New Zealand’s longest river has its origins on the eastern slopes of Mount Ruapehu as the Upper Waikato Stream. It joins the Tongariro River, which flows into the southern end of Lake Taupō. If you look at a topographic map, you can see that dozens of streams and rivers also enter Lake Taupō. Each of these waterways will have their own catchment areas too.

Lake Taupō is the largest freshwater lake in the southern hemisphere. It is so big that scientists believe that it takes about 10 years for water to move through the lake. The Waikato River exits the northern part of Lake Taupō and begins a 425 km journey to its mouth at Port Waikato, where it flows into the Tasman Sea. Numerous tributaries – including the Waipā River – empty into the Waikato River during this stretch.

Changes to rivers as they move through catchments

Rivers can change quite a bit as they move from the headwaters of their catchment to their outlets. The Waikato River is a typical example of these changes.

The upper catchment – or the headwaters – of the Waikato River is in Tongariro National Park and Kaimanawa Forest Park. The Upper Waikato Stream starts out in steep terrain at an elevation of 2,797 m above sea level. Steep streams usually have fast-flowing water and are quite narrow. Rocks and pebbles make up the substrate. As the Upper Waikato Stream leaves the mountain, it flows through native bush. The riparian canopy shades the stream and keeps the water cool throughout the year.

The Upper Waikato Stream and other small streams are tributaries of the Tongariro River, which flows into Lake Taupō. The Waikato becomes a river when it flows out of Lake Taupō. Rivers are characterised as large flowing bodies of water and are wider and deeper than streams.

Middle catchments are characterised by flatter land and slower water flow. For the Waikato River, this section is marked by the stretch from Lake Taupō to Karapiro. The elevation drops from 357 m above sea level to 22 m. The river originally ran through gorges and had lots of rapids. We’ve modified the river with hydroelectric dams, which have created sections of very slow moving and still water. The substrates of middle catchment streams are still mostly gravel and cobble, but erosion can add sediment to the water. Middle catchment streams and rivers are often missing a full canopy of plants to shade the water, so temperatures are warmer and more variable during the year.

Rivers and streams in the lower catchment often have wider, deeper channels but with slowly moving water flows. It is unlikely that the water is shaded, and it may be murky from sediment. The substrate may be silty – also due to sediment. These are characteristics of the lower Waikato River. The Waipā River is quite turbid and introduces a lot of silt as it merges with the Waikato. Downstream from Huntly, the river used to be a broad flood plain with a series of shallow lakes and wetlands, but flood control measures now prevent much of these natural flow pathways. The river forms a delta in its lowest reaches before emptying into the Tasman Sea.

He taura whiri kotahi mai anō te kopunga tai no i te pu au.
From the source to the mouth of the sea, all things are joined together as one.

Why catchments matter

Catchments influence the biodiversity and ecology of stream and river systems. Light, water temperature, pH, nutrient levels and substrate affect the plants and animals living in the water. Some of these features change naturally as water moves from the upper to lower catchments. However, human influences including land use and modifications to water flow often have more substantial impacts.