Peat lake water levels

Monitoring sites

The 31 peat lakes assessed in this indicator are:

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.

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.