Lower Clutha Rohe (Area)

The most common land cover is high-producing grassland where a lot of the intensive agriculture occurs.  

Dry stock farming consists mainly of pasture grazing beef cattle, sheep, and deer for meat, wool, and velvet production. While dry stock farming has decreased by 9%, it still remains the main land use in the Lower Clutha area and accounts for 56% of the rohe. 41% is sheep and beef; 7% mixed sheep, beef, and deer; and 8% sheep farming.  

Dairy farming occurs on approximately 17% of land and has notably increased by 37% between 1990 and 2018. 

Forestry has increased by 39% between 1990 and 2018 and now covers 9% of the rohe. Lower Clutha is about 7% conservation estate which has increased by 40% in the last 30 years.  

Soil types include Brown, Melanic, Gley, Pallic, Recent, Organic, Anthropic and Podzols. Brown and Pallic soils are the main soil types and cover 45% and 42% of the rohe. Sheep and beef farming uses a large area of the Lower Clutha and is on high-producing exotic grasslands on Brown and Pallic soils. 

Melanic soils cover 5% of the rohe and occur on ranges in parts of the Kaihiku stream, Pomahaka and Waiwera river catchments. Gley soils occur on 3% of the rohe, on alluvial deposits in lower areas. 

Lower Clutha Rohe land use summary

The most significant water feature in the Lower Clutha Rohe is the Clutha/Mata-Au River, which flows to the coast and into the Pacific Ocean downstream of Balclutha. The Clutha/Mata-Au River is the largest by area and flow and the second longest river in New Zealand. The Roxburgh power station highly modifies the flows between Roxburgh and Balclutha.  

The headwater catchments of Lakes Wānaka, Whakatipu and Hāwea cover just 34% of the total Clutha/Mata-Au catchment area but generate 75% of the Clutha Mata-Au flow measured at Balclutha. The remaining 66% of the catchment area provides just 25% of the flow at Balclutha. 

The Clutha Mata-Au has many tributaries in this rohe. The largest is the Pomahaka catchment which covers about 60% of the rohe area.  

The Pomahaka can significantly affect the flows at Balclutha when it floods. About one-third of the water used in this rohe is in the Pomahaka catchment.  

Water quality in the Lower Clutha Rohe is generally degraded with high bacteria, high nutrient concentrations and poor water clarity. High-intensity agriculture is the main land use, and drainage via tile and mole drains has been a significant source of water contamination. ORC monitors water quality and ecology of rivers, streams, and lakes. When the results are combined, they can show the health of a water body, and long-term data is analysed to show trends in water quality over time.  

Fourteen of 15 river sites we monitor for bacterial water quality failed to meet the required standard for E. coli, an indicator of faecal bacteria. About half of all river sites in this rohe also did not meet the required standard for suspended fine sediment, which indicates water clarity.  

Five sites had substantially elevated dissolved phosphorus concentrations. 25% of the sites monitored had degraded aquatic insect life (measured by the Macroinvertebrate Community Index). Lake Tuakitoto did not meet the national bottom line for chlorophyll-a (algae) and nutrients (concentrations of nitrogen and phosphorus). 

For rivers, the 20-year analysis showed degrading trends at most sites, particularly for nutrients (nitrogen and phosphorus). Our 10-year analysis indicated improving trends for most parameters across most sites in the rohe. Ten-year trends for Lake Tuakitoto show a degrading trend for phosphorus but improving trends for E. coli and suspended solids (water clarity). 

Lower Clutha Rohe Water Quality State and Trends Technical Report

Introduction to water quality

Lower Clutha Rohe water quality summary

The Lower Clutha Rohe contains the Pomahaka Alluvial Ribbon Aquifer and the Inch Clutha Gravel Aquifer, which straddles the Lower Clutha Rohe and the Catlins FMU. 

The Inch Clutha Gravel Aquifer is a potentially significant resource for groundwater, given the aquifer’s size and thickness, but generally, groundwater use is low. ORC monitoring shows that the groundwater here has high ammonia and naturally occurring arsenic compared to the NZ Drinking Water Standards (DWSNZ, 2018; ORC, 2021). 

Our bore log data indicates the water table is shallow, less than 3 metres in places on the flood plain, and 4-6 metres on elevated terraces. The Inch Clutha area has a network of drains and pumps, which lower the water table on cultivatable land and help lessen the flood hazard. The aquifer is found in the 130km2 Clutha Mata-Au Delta, an alluvial deposit-filled valley formed by interactions between the Clutha/Mata-Au River and historical sea level fluctuations. 

The Quaternary alluvium consists of gravels, sands, silts, mud, and peat. This complex geology means the Inch Clutha Aquifer has variable permeability and connections with surface water. We have recently installed groundwater monitoring bores to enable us to better understand the groundwater levels and monitor for saline intrusion into freshwater aquifers. 

The Pomahaka Alluvial Ribbon Aquifer is a series of narrow areas which follow the Pomahaka River from the upper Kelso basin to the lower Clydevale sub-basin. The gravels of the Pomahaka Alluvial Ribbon Aquifer occur along gravel boundaries deposited by the Pomahaka River. The aquifer connects to the Pomahaka River, so groundwater bores within the ribbon aquifer are assigned to surface water. 

Groundwater is often taken from fractured rock across the rohe, especially in the Pomahaka Basins and into the Catlins FMU. This water resource is not mapped as an aquifer but is a locally important water source, particularly for stock water and servicing dairy sheds. 

The Lower Clutha Rohe is home to rare and threatened ecosystems and species. The vulnerable ecosystems include ephemeral wetlands, estuaries, and wetlands. These ecosystems contribute enormously to national biodiversity, however, are often threatened by processes such as land use change and invasive species. Little is often known about the extent and/or condition of these ecosystems. 

The Lower Clutha Rohe has many species that depend on freshwater habitats and ecosystems, including fishes, invertebrates, plants, and birds. There have been 32 threatened freshwater-dependent species identified within the rohe. Threatened freshwater fishes include the Clutha flathead galaxias, gollum galaxias, Pomahaka galaxias, dusky galaxias, and lamprey.  

Freshwater invertebrates include koura and mussels, a threatened moth, caddisfly, and stoneflies. Carex strictissima and Ranunculus ternatifolius are examples of threatened freshwater-dependent plants found here.  

Many native birds depend on freshwater ecosystems, permanently or as mobile residents, including the threatened Australasian bittern, the threatened black-fronted tern and the at-risk, black-billed gull. Information is often missing at a species level, particularly for freshwater invertebrates, non-vascular plants, and algae.  

Exotic fishes include goldfish, perch and four salmonids. Many native freshwater species are under threat and continue to decline in number.  

There are 28 sites in the rohe recognised as Regionally Significant Wetlands in the Regional Plan – Water for Otago (RPW). These are classified as swamp (12 sites), marsh (7), fen (6), and bog (2). These wetlands are found within five areas: Inch Clutha, Kaitangata, Clinton, Tapanui, and Lawrence.  

On the seaward end of Inch Clutha, is the Molyneux Bay Swamp (150 hectares) which is a lagoon with swamp-edged fingers. Also in the Inch Clutha area is the Clutha/Mata-Au River Mouth Lagoon (29 hectares), an elongated water body with marsh margins; and the Clutha Matau Wetlands (21 hectares), a river-margin swamp. 

Further upriver, the Culcairn Oxbow Marsh (8 hectares) is a curved pond of a former oxbow channel, marsh-fringed, in farmland. Finegand Lagoon Marsh (6 hectares), south of Balclutha, is a stream pond with willows and rush marshes.  

Lake Tuakitoto Wetland (546 hectares) is located near Kaitangata. It is a shallow lowland lake bordered by sedge grass, rush swamp but with many crack willows. Smaller wetlands occupy fingers of stream valleys, as rush marshes, some with ponds and willows, or swamps with flax, shrubs, and red tussock. These smaller wetlands include the Frasers Stream Headwaters Marsh Complex (26 hectares); Stirling Marsh Complex (11 hectares); Camp Stream Swamp (8 hectares); Two Stone Hill Stream Swamp (5 hectares); and East Benhar Swamp (2 hectares). 

Wetland sites in the Clinton district are remnants of former copper tussock country, which are the boggiest sites. These typically contain copper tussock, wire rush, sphagnum moss, sedge grasses, some heathland, and coprosma shrubland. These are all in farmland settings: Dunvegan Fen Complex (87 hectares); Three Stones Fen Complex (58 hectares); Hazeldale Fens (10 hectares); and Willowburn Bog (4 hectares), where silver birch trees behave as weeds in the peatland. Macfarlane Road Oxbow Swamp (2 hectares) and Marana Swamp (2 hectares) are small, isolated hollows with ponds and willows.  

To the east of the Blue Mountains, John O’Groats Hill Fen (22 hectares) and Blackcleugh Burn Swamp (3 hectares) have red tussock wetlands on valley flats. The tops of the Blue Mountains, at around 900m altitude, have sphagnum moss and cushion bogs, but these are not currently listed in the RPW.  

Three small marsh sites are located on farmland near Tapanui: the Clifton Hill Marshes (4 hectares) with copper tussock, the Pomahaka River Oxbow Marshes, Dalvey (4 hectares) and Koi Creek (2 hectares), both with ponds and willows.  

North of Lawrence, Bungtown Bog (28 hectares), and partly Scientific Reserve) is a bog with sphagnum moss, wire rush, and bog pine. Glendhu Swamp (22 hectares) has valley floor copper tussock, while Malones Dam Margins (2 hectares) has a small swamp at one end.  

In the northern portion of the rohe, there are more upland wetlands (cushion bogs, snowbanks, sedge grass fens), which are not currently listed in the RPW. These are found in the headwaters of the Pomahaka by the Umbrella Range. 

The Clutha/Mata-Au River has a tidal mouth where it joins the sea. It is categorised as a ‘Shallow, short residence time tidal river (<3 days) with adjoining lagoon estuaries’. This means the water at the river mouth is replaced regularly. In these types of estuaries, the risk of contaminants building up is lower than in estuaries where longer residence times give contaminants - for example, nutrients that contribute to rapid increases in algae blooms - more time to settle on the estuary bed.  

The area at the mouth of the Clutha/Mata-Au is mainly freshwater due to the river’s flushing; therefore, most fine sediments and nutrients are exported to the sea. No estuary exists due to these features, and no limits/attributes can be measured or set for the river/coast interface. Due to the thoroughly flushed nature of the Clutha/Mata-Au River mouth, no estuary nutrient modelling has been done as all nutrients and sediments are flushed out to sea.