Publications: Research reports and publications

Observations and modelling of Seston transport in the Waitaki River downstream of the Waitaki Dam

1 June, 2005
Cawthron Report 1030. Prepared for Meridian Energy Limited.

EXECUTIVE SUMMARY

Lake outlets throughout the world are renowned for the high densities of benthic invertebrates and fish that they support. There are a variety of factors that may be responsible for this consistent pattern such as stable flows and good water quality, but the abundance of food resources appears to be the most common factor. Suspended material (seston) in lake outflow water is dominated by living organisms such as zooplankton and phytoplankton, and thus provides a much richer food resource than seston in non-outflow rivers. Filter-feeding invertebrates, such as net-spinning caddisflies, are often abundant in lake outlets owing to this high quality food resource.

Invertebrate densities typically are highest close to the lake outlet and decline rapidly downstream. This pattern appears to be related to a downstream decline in the concentration and quality of seston. As it moves downstream, seston is lost through settling, filtration through river-bed sediments and consumption by filter feeding invertebrates. The length of river over which seston is transported depends on flow and channel morphology. Deep fast rivers transport seston further than shallow slow rivers.

Abstraction of flow from the Waitaki River downstream of the Waitaki Dam has the potential to reduce the length of river that receives high-quality seston from the lakes upstream with potential consequences for ecosystem productivity. This report describes longitudinal patterns in seston concentration downstream from the Waitaki Dam and predicts the effect of flow change on the distance seston is transported downstream.

Seston was sampled at four sites on the Waitaki River between the Waitaki Dam and just downstream of Kurow (7.4 km downstream). Concentrations of coarse seston (>125 μm) were highest close to the dam and declined downstream during the day. Concentrations were also highest close to the dam at night, but flow variations during sampling confounded these results. Fine seston concentrations were similar at the three upstream sites during both day and night, but were significantly lower at the most downstream site below Kurow.

An exponential removal model gave a reasonable match between observed and predicted declines in seston concentration. Seston deposition was faster at low flows. For example, at 350 m3/s lake-derived seston was predicted to remain in suspension over at least the first10 km downstream of the dam. However, at 100 m3/s little of it was predicted to remainsuspended after 3 km. A permanent reduction in flow from 350 m3/s to 100 m3/s wouldsignificantly reduce, or eliminate, the energy subsidy from lake seston for the river ecosystem 3-10 km downstream from the dam.

This reduction in potential food supply could reduce the growth rates, size and density of filter-feeding invertebrates over this reach if they are food limited. We consider this is likely given that high densities of filter feeding caddisflies have been recorded in the first 8-10 km of the Waitaki River downstream of the Waitaki Dam, but not further downstream. However, other factors such as differences in sediment transport and physical habitat quality downstream may also contribute to this pattern. Caddisflies dominate the invertebrate community and constitute the major component of trout diet in the Waitaki River around Kurow. Food supplies are considered to have an important role in controlling fish numbers and growth. Therefore, a reduction in the length of river that is subsidised by lake-derived seston, caused by flow abstraction, could reduce trout growth and carrying capacity in this reach.