Monitored sites in the Ellesmere / Waihora Catchment catchment
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Waihora/ Lake Ellesmere, covering a total catchment area of 26708 km², is also one of Environment Canterbury’s new water management zones. The catchment is diverse in terms of its representation of riverine and lake environments. The Selwyn River is the main inflowing river, which is itself fed by key tributaries such as the Hororata, Hawkins and Waianiwaniwa rivers.
There are also a large number of inflowing smaller spring-fed tributaries that reflect hydrological gains from aquifers that dominate the Canterbury Plains. Several inflowing tributaries, such as Kaituna Stream, are sourced in the volcanic geology of Banks Peninsula.
Te Waihora/ Lake Ellesmere is the fourth largest lake in New Zealand. Its shallow depth – which averages 1.4m – and exposed location, causes highly turbid conditions because of wind-induced wave action and re-suspension of lake-bed sediment. Although the lakes outlet to the sea is closed by a 26km long sand/ gravel, groundwater intrusion of salt water means that the lake is brackish. However, there is a decreasing salinity gradient northwards due mainly to the predominant contribution from freshwater groundwater. The lakes margins are dominated by salt-marsh vegetation of national importance. Indeed, the lake is recognized internationally for its abundance and diversity of wildlife and a water conservation order was granted in 1990.
Te Waihora has long been of strong cultural significance to Maori because of its bountiful resources and mahinga kai. For example, the tuna/ eel and whitebait fisheries were of key importance, as were the numerous waterfowl species that used the extensive adjacent wetlands. However, since the mid- 1800’s, these wetlands have been in continual decline due to drainage and clearing of vegetation for farming. Although historically the lake spilled to the sea during high flows, this is no longer the case. The lake level has been managed since the late 1800’s by artificial openings to reduce the lake’s level and the risk of flooding. These openings are now managed by Environment Canterbury and their timing is critical in order to enable peak migrations of eel, whitebait and salmonids.
Landuse and vegetation cover in the upper catchment is predominantly dryland and tussock, with low intensity grazing. Although large wetlands occur in the upper catchment, these now occupy an area that is a remnant of their historical distribution. In the lower plains, moderate to high intensity sheep, beef and dairying now dominate the landuse. Urbanisation is also increasing with a human population of approximately 38000. Environment Canterbury includes 18 sites in its long-term water quality monitoring programme, with the distribution representative of hill-fed, spring-fed and volcanic rivers, as well as lake sampling sites.
The upper Selwyn river at Whitecliffs has generally good water quality, with low nutrient levels reflecting the low intensity surrounding landuse. However, in major tributaries such as the Waianiwaniwa and Hawkins, nutrient levels and faecal indicators are much higher. In lowland spring-fed tributaries such as the LII, Irwell and Halswell rivers, these contaminants are even higher; high nitrate concentrations reflect the influence of landuse activities further upstream in the mid-catchment, and the subsequent leaching to groundwater. High phosphate concentrations are mainly due to adjacent landuse activities and run-off associated with sediment-bound particles. However, increasing pressure for water abstraction leading to low flows has exacerbated these effects by reducing the capacity for contaminant dilution.
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