Roxburgh Rohe land use summary

heavy rain to the west. In the east, much lighter rain falls due to the sheltering effect of the Southern Alps. This effect is typical in Central Otago. 

A mountain range on the western border also provides shelter from rain coming from the southwest. Heavier rain from the southwest falls at the tops of this mountain range, but rainfall quickly decreases towards the east. 

The Roxburgh Rohe is in the heart of Central Otago and subject to typical weather conditions for this area with hot, dry summers and cold, frosty, dry winters. Mean annual rainfall is about 1200mm on the Obelisk/Old Man Mountain ranges, around 900mm on the hills south of the mountains, about 360mm near Alexandra, and 450-500mm further south. 

Temperatures can range from highs of more than 38°C in summer to lows around -10°C in winter. Evaporation is very high, especially in the lowlands, where it usually exceeds precipitation, and creates a moisture deficit. 

Rivers and streams originating in this rohe do not have large flows and generally have very low flows in summer. However, the Clutha/Mata-Au River, which runs through the centre of this rohe, has a healthy flow throughout the year. 

All the rivers and streams in this rohe are used extensively for irrigation due to the extreme dryness in the spring, summer and autumn. In summer, the smaller streams and rivers can run dry due to both natural losses to groundwater and water taken for irrigation. Hydroelectricity generation at Roxburgh Dam is also an important control on water quantity. 

Water quality in the Roxburgh Rohe is generally good. However, there are signs of degraded water quality in some indicators we measure, particularly in the Benger Burn. Potential pressures on water quality include pastoral farming, orcharding and plantation forestry. At times, these stresses on water quality may be made worse by low flows in tributaries. 

We monitor water quality and the ecology of rivers and streams at Teviot River, Fraser River, the Clutha/Mata-Au River at Millers Flat and Benger Burn. Results indicate the health of a river or stream, and we analyse long-term data to see trends. We have been monitoring some sites in this rohe for fewer than five years, so only have interim results for the current water quality state and trends. 

Monitoring results show the Benger Burn, Clutha/Mata-Au and Teviot River sites did not meet the required national standard (National Policy Statement for Freshwater Management 2020) for E. coli (human health indicator) and suspended fine sediment (visual clarity). 

For the river sites, only the Clutha/Mata-Au River at Millers Flat has enough data for trend analysis. The analysis for this site shows likely improvements in nutrients over 20 years but decreased visual clarity. 

The results for Lake Onslow are mixed, with degrading trends over ten years for ammoniacal nitrogen and chlorophyll-a (a measure of algae in the water) but improving trends for visual clarity. Ammoniacal nitrogen can enter waterways via effluent and sewage and is toxic to aquatic life at high concentrations. 

The area contains several recognised groundwater basins and associated aquifers. The Alexandra basin in the north includes the Dunstan Flats, Earnscleugh Terrace and some of the Manuherekia Claybound aquifers. There are also the Roxburgh (Roxburgh East & West aquifers) and Ettrick basins. 

The aquifers are mainly found in glacial outwash deposits. The geology of the aquifers and key parameters such as how far down the water table is and how good the aquifer is at transmitting groundwater vary greatly. The interaction between groundwater and surface waterbodies also vary across the rohe, being different for the Benger Burn, Fraser, Clutha Mata-Au, and Manuherekia rivers. 

Water from surface water bodies and irrigation schemes recharges groundwater, which is primarily used for domestic consumption, stock water, community supply and irrigation. Areas like Ettrick are developing rapidly and seeing changes in land use. This is likely to put pressure on the quality and availability of groundwater. 

Groundwater quality results vary. Results from two ORC monitoring bores in the Alexandra basin indicate good groundwater quality, with no exceedances of the NZ Drinking Water Standards for arsenic, E. coli or nitrate. 

Results from the Roxburgh basin show no exceedances of the E. coli or arsenic limits. However, nitrate concentrations are between 33% and 50% of the limit (11.3mg/L). This suggests that it is wise to monitor nitrate concentrations in this area. 

Results from two monitoring bores in Ettrick are poorer than the other areas, with several E. coli exceedances in both bores. Groundwater nitrate concentrations are also high, ranging between around 50% and 85% of the limit. Dissolved arsenic limits were not exceeded in either bore. This area’s rapid land use changes and development will likely worsen these issues. 

The Roxburgh Rohe has extensive freshwater biodiversity features including rare and threatened species, and vulnerable ecosystems including braided rivers, ephemeral wetlands, inland saline, lake margins and wetlands. These ecosystems are often threatened by changes in land use and invasive species. We know little about the extent and/or condition of many of these ecosystems. 

Diverse species depend on freshwater habitats and ecosystems, including fishes, invertebrates, plants and birds. We have identified 40 threatened freshwater-dependent species in the Roxburgh Rohe. Threatened freshwater fishes are the Clutha flathead galaxias, Teviot flathead galaxias, dusky galaxias and lamprey. 

Threatened freshwater invertebrates are a moth, a true bug and a stonefly. Threatened freshwater-dependent plants are Triglochin palustris and Crassula multicaulis. 

Many native birds depend on freshwater ecosystems, permanently or as mobile residents, including the threatened Australasian crested grebe and black-fronted tern. We lack information about species, particularly freshwater invertebrates, non-vascular plants and algae. 

Exotic fishes found in the rohe are perch and three salmonids. Many native freshwater species are under threat and their numbers continue to decline. 

Twelve sites are mapped as Regionally Significant Wetlands: these are classified as inland saline (4 sites), ephemeral wetland (1), fen (3) and marsh (4). Wetlands are common in upland areas, with copper tussock, sedgeland and herb field the most widespread plant types in this rohe. 

The streams draining into Lake Onslow from the south (Boundary and Fortification Creek) are perhaps the most distinctive and impressive upland wetlands in Otago, if not nationally. Northern parts of the rohe, on the margins of the Manuherekia, support several saline wetlands. 

Inland saline sites are in the Conroys Gully area: Conroys Dam Inland Saline Wetland Management Area (18 hectares), Conroys Road Inland Saline Wetland Complex (seven hectares), Chapman Road Inland Saline Area (seven hectares), and Blackmans Inland Saline Wetland Management Area (12 hectares). 

They are mainly towards the bottom of hillside slopes, intermittently wet with seepage from groundwater. These areas have saline and alkaline soils and support salt-tolerant plants that otherwise occur in coastal salt marshes. They also have some annual species that are dormant in dry seasons, which is unusual in native plants. 

Flat Top Hill Ephemeral Wetlands (five hectares) form on surface depressions of hill-crest plateaus. These wetlands also have native annual plants that tolerate both ponding and drought. 

The most extensive wetlands in the rohe occupy the valleys that feed Lake Onslow from the south. These wetlands are Fortification Creek Wetland Management Area (526 hectares, including the Teviot River South Branch), Boundary Creek Fen (94 hectares), and Middle Swamp (67 hectares). In these fen wetlands, valley floors with gentle gradients have developed meandering systems on a smaller scale and at a higher altitude than the broad scroll plains of the upper Taieri. 

These have complex patterns of sinuous stream channels, cut-offs, oxbows and old river channels. Copper tussock grassland grows in the alluvial flats, along with sedgelands, turf communities in the hollows, aquatic plants and sphagnum fens at the valley sides. 

We have identified four small marsh sites in the Ettrick to Roxburgh area: Island Block Pond Marshes (four hectares), Upper Black Stream Marshes (three hectares), Rigney Pond Marshes (0.5 hectares), and Gilmour Road Marsh (one hectare). 

A significant wetland not currently listed in the RPW is Teviot Swamp. This is a large fen complex found in a basin at the head of the south branch of the Teviot River at 1000m altitude. Fed by groundwater and seepages from the top of the Lammerlaw Range, sphagnum moss is the main peat-forming plant on the wettest ground. 

As water movement changes with time, the vegetation changes to cushion plants. In the uppermost tributaries, moss and cushion communities are part of small string bog systems, with pools in terraces. Other fingers of valley wetlands at upper altitudes also occur in the heads of the Fraser River, west of Alexandra.