Flow in the Waikato region
River flows change naturally, mostly because of changes in rainfall and evaporation. Artificial changes to rivers, such as dams, diversions, culverts and land use also affect flow. River flow must be managed carefully to ensure enough water is available for human use while maintaining natural habitat.
Water is essential for our everyday lives and for the plants and animals that live in our rivers and streams. In total approximately 82,000 litres of water is used per day for every person in New Zealand (this includes water for electricity generation, irrigation, stock watering and personal care).
How much, how hard and how regularly it rains influences river flow. The Waikato region’s average annual rainfall is 1,250 mm, ranging from:
- 1,100 mm in the Hauraki Plains
- 3,500 mm in the Coromandel Ranges and south of Taupo
- about 2,500 mm on the western ranges from Mokau to Port Waikato
- over 5,000 mm at Mt Ruapehu.
Low flow varies across the region. The most consistent flow is found in:
- spring fed streams of the Hauraki basin and Taupo area
- rivers flowing from lakes.
Rivers with little variation in flow usually:
- are large and can hold large volumes of water in their channels
- have few, small floods
- support many aquatic plants and animals.
High flow occurs in areas of high rainfall and steep slopes. These areas often experience large variation in flows, and frequent flooding.
Find out more about the size and variation of average annual flow for selected catchments.
The quantity and flow of water in the region’s surface waters is affected by:
- the amount of water taken
- dams and diversions
- culverts and fords
- land use.
Most surface water takes are drawn from sources located close to towns and cities, placing high demand on local streams and rivers.
In some areas such as Pukekohe, surface water availability varies and peak demand often coincides with periods of low flow when the weather is dry. This often results in water demand exceeding the amount available in local streams. Careful management is needed to ensure the long-term health and sustainability of Pukekohe streams.
Waikato Regional Council generally requires resource consents for surface water takes that are
- greater than 15 m3/day
- for uses other than stock watering, domestic needs or fire fighting.
Consent conditions set maximum rates for water takes and minimum flows for dams and diversions. They may also include requirements for:
- riparian management
- fish passes
- bed stabilisation works.
In our region, resource consents authorise the taking of nearly 250.6 million m3 of surface water per day. Nearly all of this water is used for non-consumptive purposes, such as cooling water for industry. The rest is for consumptive use such as irrigation and public water supply.
Dams and diversions
Dams and diversions are common throughout the region. Dams range in size from small farm dams to the large hydro dams on the Waikato River. Diversions include stopbanks, weirs (to regulate water flow) and groynes (low walls built to stop erosion).
Very large dams and diversions are used to generate electricity, but they disrupt the natural flow of a river. The eight hydro dams on the Waikato River affect:
- natural flow patterns
- the movement of fish up and downstream
- the movement of river sediments
- water quality.
Many of the adverse effects of these dams are offset by mitigation measures set by consent conditions and undertaken by the dam owners.
Culverts and fords
Culverts and fords are placed in river and stream beds to create permanent crossings. They range in diameter from 10 cm for small stream crossings to six metres for major highways.
The use of culverts on farms, subdivisions, road networks and other land development practices has become more common over the past decade. If not properly maintained they can:
- cause erosion of the stream bed
- change a stream’s natural flow
- prevent or inhibit normal fish passage
- reduce flood capacity.
The combined effect of many small but poorly constructed culverts on rivers and streams can be far greater than one or two large structures.
Waikato Regional Council monitors water level, flow and rainfall at water monitoring stations around the region.
To protect surface water resources, we set upper limits on how much surface water can be taken. These limits are based on the flow and what aquatic plants and animals are present in each river or stream.
In some areas water take restrictions are applied during low flow conditions to protect water resources from over use. Restrictions may include ‘no take days’ when consent holders are asked not to take water from their waterway so that we can measure stream flows more accurately before more stringent restrictions are imposed.
Environmental education is also used to encourage landowners and water users to review their water requirements and assess how they can use water more efficiently.