By Sarah Mager
The Southern Alps of New Zealand are the source of some of New Zealand’s most iconic river systems. The development of the South Island has been intimately connected with these powerful water sources. For instance, the Clutha initially provided a critical navigation route into Central Otago and the early gold fields; the Waitaki sustains eight hydroelectrical power stations that were crucial to developing a power-hungry growing nation; the Waimakariri, Rakaia and Rangitata all provide large water sources to the Canterbury Plains, both as surface and groundwater resources, and are critical to pastoral development and the rapid expansion of dairy production in the region. All of these rivers are driven by the interplay between atmospheric processes and the resultant uplift along the tectonic plate boundary. In geological terms, the Southern Alps are one of the fastest uplifting mountain ranges in the past 500 millions years. The rapid uplift impedes air flow, causing the upward deflection of humid air masses from the Tasman Sea and produces 5+ m a year of rain to the rolling foothills of Westland, and >10 m a year of water equivalent rain onto the main divide.
The uplift of the Southern Alps due to the active plate margin is offset by the relentless processes of erosion that wear down mountains at a rate almost equal to that of uplift. The balancing of uplift and erosion processes in the Southern Alps means that the rivers that drain the Southern Alps are characterised by high rainfall and high sediment – the two key factors that contributes to their spectacular geomorphology and hydrology. The large braided rivers of Southland and Canterbury are a function of the redistribution of the material eroded from the Southern Alps by glaciers, earthquakes, rock falls, avalanches and streams. It is evident from the large gravel-filled riverbeds that much sediment is redistributed through the network of rivers that drain both the east and west of the main divide of the Southern Alps. However, alongside the pulsing movement of bed load (that is the gravel and coarse material), there are finer fractions of rock material also being transported, either as suspended or dissolved load. The mountains are continually being reshaped by erosion processes due to the relentless delivery of ice, snow and rain in the Southern Alps. Freshly exposed rock faces are subject to pelting rain and frost fracture, weakening the mineral grains and making it possible for chemical weathering processes to occur. The soluble and reactive minerals are exchanged from the rock surface and dissolve into water. Their presence in rivers and lakes along the Southern Alps helps to attenuate refracted light producing striking intensely coloured water bodies.
The weathering of minerals from mountain ranges play an important role in global biogeochemical cycles, that is, the chemical reactions that occur as minerals like silica, calcium and magnesium are released from rock faces, consumes carbon dioxide from the atmosphere. The drawdown of atmospheric carbon by eroding mountain ranges has garnered significant attention over the past decade, particularly to understanding how atmospheric carbon dioxide is mediated over geological time by active tectonics and weathering processes. Mountainous rivers, like those in the Southern Alps, play an important role in the silicate weathering cycle, as these catchments have very high erosion rates. For example, a study into the chemical weathering rates in the Southern Alps showed that streams with high physical erosion and extremely wet climates contained between 10 to 25% more products from weathering compared to drier and more stable landscapes (Jacobson et al, 2003). Thus, the presence of a rapidly uplifting mountain range is potentially significant carbon sink through the drawdown of carbon dioxide by chemical weathering processes. There are, of course, other carbon sinks at play in the Southern Alps, including the sequestration of organic carbon from plants, soils and eroded bedrock to offshore or deep lake storage. In areas of rapid uplift in coastal regions, there is a significant flux of organic carbon transported through river systems and may be of greater significance than carbon drawdown from silicate weathering. Quantifying how much organic carbon is transported from the Southern Alps is tricky since the movement of organic carbon from mountain catchments is mostly during storm events. Thus, the combined effects of carbon drawdown via chemical weathering and the cycling and burial of organic carbon from soils and native forest cover in the Southern Alps mean that this landscape may be a regionally significant carbon sink and contribute in a small way to offsetting global atmospheric carbon dioxide concentrations.
Dr Sarah Mager is an Senior Lecturer in the Department of Geography, University of Otago.