Biodiversity in a disappearing river

Disappearing flows in the middle reaches of the Selwyn River. Photos were taken at the same location in November 2003 (top) and February 2004 (bottom).

NIWA scientists are studying the environmental consequences of extreme low flows in the Selwyn River, south of Christchurch. Of particular interest are the effects of seasonal drying and wetting of the riverbed on aquatic biodiversity values.

The Selwyn River drains the central Canterbury foothills, and flows – sometimes – for 60 km across the Canterbury Plains to Lake Ellesmere. Its most striking feature is the absence of water in its middle reaches for most of the year. Where does all the water go? How does this influence biodiversity?

In the foothills, the Selwyn flows year-round. On the plains, the riverbed is highly permeable, and the river overlays a deep and porous aquifer. As soon as the river reaches the plains, water begins leaking down through the bed and into the aquifer. In most months, all river water disappears within 5 km of leaving the foothills. The next 35 km of the river is dry for most of the year, apart from a small section around the confluence with the permanently flowing Hororata River. About 15 km upstream from Lake Ellesmere, shallow groundwater rises back to the surface, and the Selwyn becomes permanent again.

Disappearing river flows have significant ecological effects: when the river’s surface water disappears, so does the habitat for many aquatic plants and animals. In response to loss of surface water, aquatic invertebrates and fish must disperse, seek refuge in remnant aquatic habitats, or die. Aquatic plants, algae, and bacteria must form resting stages, or die. The dry central reaches of the Selwyn River also constitute a significant barrier for dispersal of invertebrates and for fish migrating between Lake Ellesmere and the headwaters.

The environmental effects of changing flows in the Selwyn River are the topics of a new long-term FRST-funded research project being conducted by NIWA and Lincoln Ventures Ltd. We chose the Selwyn River for our research because it responds strongly to changes in runoff from the foothills, and to changing groundwater levels. The Selwyn is also a ‘sentinel site’ for predicting future conditions in catchments where land-use is shifting toward intensive agriculture with high irrigation requirements.

The first stages of our research focus on patterns in surface water chemistry, benthic invertebrates, and attached algae and plants, and hyporheic (interstitial) invertebrates and chemistry. We collect samples monthly at 14 river reaches down the length of the Selwyn. Results from November 2003, when the entire river was flowing, indicate that longitudinal patterns in invertebrate communities are strongly influenced by flow permanence. Several taxa occur only at sites with permanent flow. These include the large predatory stonefly Stenoperla prasina, the dobsonfly Archichauliodes diversus, and several cased-caddisflies. In contrast, the mayfly Deleatidium, midges (Chironomidae), and oligochaete worms occur at all sites. The former taxa have long larval periods, which make them susceptible to periodic drying, whereas the latter taxa tend to complete river-dependent stages of their life-histories during short periods of flow. We also found that invertebrates characteristic of groundwater habitats (stygofauna) are common in the riverbed where groundwater upwells, such as the deep remnant pools adjacent to the Selwyn’s main channel. Future research will use these facultative groundwater taxa to examine surface-subsurface exchange of water and organisms.

Mike Scarsbrook, Scott Larned, Graham Fenwick, and Dave Kelly