Constructed on farmland owned by Chris Dagg, using Department of Conservation’s Jobs For Nature funding, the trap — the length of an Olympic swimming pool and about 20m wide — aims to decrease the amount of sediment flowing via Mill Creek into the lake.
Dubbed "Pukunui", it is the larger of two sediment traps in the area — the smaller "Pukuiti" is downstream.
Mana Tāhuna environmental projects manager Sarah Mukai said the larger trap , designed by e3Scientific and approved by the Otago Regional Council, allowed the water to separate from sediment, slow down and carry on to the lake, while the sediment settled in the man-made pond.
Until now, the nutrient-rich sediment had been allowed to flow largely unconstrained into the lake and, once there, those nutrients fuelled the toxic algal blooms.
"We know it [the trap] is going to work, because when we have a drought year we naturally have less rainfall, therefore less sediment, we see those improvements [in the lake health]," she said.
"The last two years, we’ve not had the toxic algae because of the lower rainfall on average."
Ms Mukai said even reducing the amount of sediment going into the lake by 20% would be enough to turn the water quality around and keep it clean.
Chief executive Michael Rewi said unlike the riparian strips, which took a long time for results to be seen, the sediment traps would provide a "real impact".
Water testing would continue in the wetlands at the northern end of Lake Hayes, and both up and downstream of the sediment traps, while the amount of sediment would be monitored.
Once the trap was full — which could take about two years — it would be emptied out and stored on the site.
However, Mr Rewi said the material would be sent for testing to find alternative uses.
In the North Island, for example, sediment had been used in concrete, or it could be used as fertiliser.
At the celebration, Friends of Lake Hayes member Andrew Davis said the group was "extremely appreciative" to Mr Dagg, describing it as a "fantastic gift to the community".
He said restoring the health of Lake Hayes was a "whole of the community" job.
