Environment Canterbury chief scientist Tim Davie explains how tuna (eels) became stranded in the Selwyn River and why flows are so low
The Selwyn/Waikirikiri River is a classic Canterbury Plains river with headwaters fed from rainfall in the foothills, mid-reaches which frequently dry up and spring-fed lower reaches which flow constantly.
Ngāi Tahu ancestors eloquently described the river as waikirikiri, meaning 'river of gravel or stones', confirming that the mid-reaches have always dried up as surface water becomes groundwater.
In mid November there was substantial rainfall in the Selwyn headwaters. This caused a small flood wave to move across the plains, but it did not reach the bottom.
The flood wave travelled from Whitecliffs to Greendale, but it did not reach Coes Ford. Tuna in the upper reaches used the flood wave to move downstream. They did not reach the coast and became stranded in the mid-reaches. As the flows in the headwaters subsided, the flowing part started to retreat, stranding more fish.
In the lower reaches, flows are very low due to extremely low groundwater levels. Every month since July this year has seen record low flows.
Groundwater gains water (recharge) from two sources: water infiltrating from rivers as they cross the plains, and rainfall infiltrating past the soil.
Most rainfall recharge happens in winter when soils are wet, evapotranspiration is low and rainfall moves rapidly to groundwater. The two most recent winters have been extremely dry, meaning very little rainfall recharge.
Groundwater loses water (discharges) through springs as part of groundwater fed streams and through extraction for irrigation and drinking water.
When the inputs (rainfall and streamflow recharge) are lower than the outputs (spring-fed streams and human extraction) groundwater levels drop - and vice versa. This causes regular rising and falling of groundwater levels.
Because rainfall recharge is the largest input into the groundwater system, a series of dry winters causes very low groundwater levels. Groundwater extraction causes some of the low levels, but it is relatively insignificant.
The last time we saw groundwater levels as low as they are now was in the summer of 2005-06. The lowest ever flow at Coes Ford was in March 2006 (20 litres per second). The flow there in early December this year was 115 litres per second, equalling the lowest recorded in December 2005.
The situation in 2005-06 was broken by snow in June 2006, which provided a large groundwater recharge boost.
There are full irrigation restrictions on all takes from the Selwyn River. Many groundwater takes are on partial restriction and all deep groundwater takes have restrictions on their annual volumes. This has been the situation since November 2015.
When you see irrigators working they will be using water sourced from winter storage, deep groundwater or large alpine rivers which currently have good flows.