Peat lake water levels

Why we monitor peat lake water levels

The region’s 31 peat lakes are concentrated around the Waikato and Waipa Districts, and Hamilton City (though one is found in Otorohanga District) where their association with peat formations has had a marked effect on their physical, chemical and biological nature.

The Waikato peat lakes form the largest collection of peat lake habitat in New Zealand and are valuable conservation refuges for many unique plant and animal species. They represent some of the few remaining areas of wetland in the formally extensive Komakorau, Rukuhia and Moanatuatua peat bogs. The Waikato peat lakes are valued for their unique genetic diversity, cultural and spiritual importance, scientific interest, recreational opportunities and intrinsic value.

The development of peat bogs for agriculture has adversely affected water quality and quantity, as well as biodiversity and natural character of almost all peat lakes.

The Waikato peat lakes are particularly vulnerable to water level lowering. In the past, much of the wetland area in the region was drained to provide farmland for agricultural purposes. Drainage and cultivation of peat is essential to establish productive pastures and crops, but it results in irreversible shrinkage and oxidation of peat soils.

Continued deepening of drainage systems is often required to maintain the productive capacity of the peat soils. Where excessive shrinkage occurs adjacent to the sensitive peat lakes, lake water can ‘spill out’ onto surrounding farmland unless outlet drains are lowered. As a consequence, the lakes are now much shallower and occupy far less surface area than they did in the past. Shallower lakes are more exposed to environmental variables that can lower water clarity (for example, increased wind stir re-suspending bottom sediments).

The installation of water control structures in high risk peat lakes is a crucial step to ensure the sustainability of these ecosystems.

What's happening?

Before Europeans settled, most of the peat lakes in the Waikato region would have had no inlets or outlets. Instead, seasonal influences (for example, rainstorms or droughts) were responsible for variations in lake levels. Water probably entered the lakes from adjacent peat bogs and rainfall, and was lost mainly through evaporation.

The Waikato region’s peat lakes are vulnerable to the lowering of water levels. Changes in land use, particularly land clearance and drainage, have resulted in most peat lakes becoming very shallow. Uncontrolled drainage could even result in them disappearing completely.

Today, Waikato Regional Council is working with other Waipa Peat Lakes and Wetlands Accord signatories to protect the minimum water levels of peat lakes.

Members of the Waipa Peat Lakes and Wetlands Accord include:

Waipa District Council
Ngaa Iwi Toopu o Waipa
Department of Conservation
Auckland Waikato Fish and Game Council, and
Waikato Regional Council.

Structures are being installed in peat lakes where there is a high risk of their water level being lowered. These structures (culverts or weirs) help control the water leaving the peat lakes.

There are 31 peat lakes within the region (view our map) of which 19 do not have water level control structures. All peat lakes, including those with control structures, have been assessed in terms of the risk of their water level dropping. This risk is determined as high, medium or low.

Methods - how we monitor

Where and how we collect the data

Monitoring sites

The 31 peat lakes assessed in this indicator are:

Waipa District	Waikato District	Hamilton City	Otorohanga District
Henderson Pond	Lake Whakatangi	Horseshoe Lake	Lake Rotongata
Lake Kareaotahi	Lake Areare	Lake Rotoroa
Lake Koromatua	Lake Hotoananga
Lake Mangahia	Lake Kainui
Lake Mangakaware	Lake Kaituna
Lake Maratoto	Lake Komakorau
Lake Milicich	Lake Pikopiko
Lake Ngaroto	Lake Rotokauri
Lake Ngarotoiti	Lake Rotokawau
Lake Pataka	Lake Tunawhakapeka
Lake Posa
Lake Rotomanuka North
Lake Rotomanuka South
Lake Rotopiko East
Lake Rotopiko North
Lake Rotopiko South
Lake Rotopotaka
Lake Ruatuna

Monitoring frequency

Waikato Regional Council regularly collects new information as control structures are installed, through the consent process or through information exchange forums such as the Waikato Wetland Forum or the Waipa Peat Lakes and Wetlands Accord.

Monitoring history

Field surveys by the Department of Conservation recording the presence of control structures, and the risk of lake level decline were undertaken in February 1998. Financial assistance was provided by Waikato Regional Council, Waipa District Council and the Waikato District Council.

Measurement technique

Information about water level control at each lake was gathered. A field sheet was compiled that noted:

the presence or absence of any engineered control structure such as a weir or culvert
any constriction in the outlet channel likely to be restricting flow
the profile of an engineered structure and the invert (bottom of the water course) level.
where a structure was absent, the longitudinal profile of the lake outlet and the substrate type.

Additional information relevant to the project was also noted such as the location, size and invert level of road culverts, the proximity to other lakes with ‘fixed’ water levels, and the water level within the outlet at the time of survey.

Waikato Regional Council determined the legal status of any water level control structure from consent information.

Lakes Mangahia, Rotomanuka North, Areare, and Maratoto had automatic water level recorders installed between 1998 and 2001. These devices record water levels every 30 minutes and will allow lake levels to be tracked long-term. Additional recorders are to be installed in other Waikato peat lakes.

The field survey information was entered into a database.
An assessment of the risk of the lake level being lowered was made by:
- determining the legal status of a structure
- recording the construction and permanency of a structure
- determining whether the lake was in private or Crown ownership
- noting other factors which Environment Waikato has control over
- using professional judgement.
Lakes were assigned a risk status of high, medium or low depending on the outcome of the above assessment. For example:
A Crown administered lake has no engineered control structure, but its minimum water level is defined by an adjoining wetland, which it spills into during periods of medium to high lake levels. In the past an artificial outlet was constructed through which the wetland drained this lake. This has since silted up and no longer functions and the lake’s water level has returned to a more natural state. It is possible that the drain could be re-opened. However, this would require a resource consent that Environment Waikato could decline or impose conditions on. Therefore, because it is unlikely that the drain would be re-opened without mitigation of any adverse effects, this lake is classified as low risk.
Another nearby lake has its water level controlled by an old board weir. This construction is nearing the end of its useful life and it is likely that it will fail in the near future. Until the structure is replaced this lake is classified as high risk.
None relevant to this indicator.
The use of professional judgement in assigning risk categories could result in some inconsistency if the process is not clear. This is not considered a major limitation as the principle objective of this indicator is to describe over time the transition of lakes with no structure and at high risk, to lakes with structures and at low risk.

Results - data and trends

The graph shows that 53 per cent of Waikato peat lakes without structures (10 out of 19) are considered at high risk compared to 25 per cent of lakes with a structure (3 out of 12). Also, 67 per cent of lakes with structures installed (8 out of 12) are considered at low risk compared to only 16 per cent of lakes without structures (3 out of 19). The data was updated in 2005.

Download the data file

This Excel spreadsheet contains the source data to this indicator's graph and any additional data.

Download the data file
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Useful links

You can view a pdf file of a case study on the Lake Ngaroto restoration project undertaken by the Waipa District Council on the Ministry for the Environment’s website.

Documents available from Waikato Regional Council

You can order any of these documents from our library. Most documents will incur a charge.

Barnes, G.E., 2002. Water quality trends in selected shallow lakes in the Waikato region, 1995-2001. Environment Waikato Technical Series 2002/11, Hamilton.

Barnes, G.E., 2002. Water quality trends in Lake Rotomanuka North. Environment Waikato Technical Series 2002/03.

Barnes, G.E., 2001. Aquatic and marginal vegetation of Lake Serpentine North. Environment Waikato Technical Series 2001/03, Hamilton.

Environment Waikato, 1999: For peat’s sake. Good management practices for Waikato peat farmers. Environment Waikato, Hamilton. 28p.

The following are Department of Conservation publications:

Cromarty, P., Scott, D.A. (eds.) 1996: A directory of wetlands in New Zealand. Department of Conservation, Wellington, New Zealand.

When this indicator is updated

This indicator will be updated annually.