The power of three: or how three weirs are better than one

Taking even the most cursory look through the pages of history, science, art, philosophy, religion, sport and mathematics, there’s no denying that three is a powerful number. After seeing firsthand how well two weirs worked to filter the water leaving the farm, we set about creating a third – marking a final chapter (for now) in our Southern Catchment Treatment work.

Back in January of 2017 we created our first two weirs, by moving the earth right by the exit of one of the streams on the farm. In essence, our plan was to put the stream back to how it was before it was dredged in the 1960s. As with so many farms during that time, this was done to make room for more pasture and get rid of ‘boggy’ areas. 

By raising the water levels in the stream, we increased the amount of time the water is on the farm. Over time, this will have several benefits: it will improve the efficiency of the removal of suspended sediment; help the denitrification process; and scale back the presence of nitrogen, phosphorus, copper and zinc by perhaps 75 per cent. Faecal pathogen levels, too, are expected to decrease naturally by 80–90 per cent through increased exposure to the sunlight and microbial plant communities.

It was just over a year after we created our first weirs that we decided to make the third, with more generous assistance from The Waikato River Authority. Adjacent to the first two was an area known as a seep. It hadn’t been dredged, and a lot of raupō were naturally growing among it. Given its proximity to the existing weirs, and the fact that it gets boggy, especially in winter, the decision to turn it back into a working wetland was an easy one.

Making a weir isn’t just a simple case of plonking down rocks to slow the water flow. We had an on-site meeting with our engineer and the digger to make sure the weirs would work well, flowing at the right speed, during every season of the year. The two ducks that moved in shortly after the first two weirs were installed also had a thing or two to say about it. In the excavation process for the third weir, the soil got turfed up and took a week, or so to settle back down again and for the water to get back to its normal shade. 

Last week, we worked with a local fencer to section off this new area from stock. As with all our new fencing on the farm, we use reclaimed concrete posts. One of the regulations around maintaining an organic certification is that you don’t use tanalised timber, as the chemical preservatives can seep from it and into the soil. Up until this point, the area was sectioned off using a more temporary electric fence reel and standards. As with the other two weirs, we needed to see where the water would naturally rise to before we formally sectioned it off. This process allows us to plant generous riparian areas, in addition to the core wetland plants.

Over the remaining two months of winter we’ll be planting it up with “eco” vegetation – plants that have been carefully selected and locally sourced. Due to the biosecurity risk of myrtle rust, we have been operating a mānuka nursery for two years. This involves collecting seeds from our existing plants, cultivating them and planting them into trays. Our planting pattern and plant list will be the same as with the other weirs, to ensure the area works together as one large treatment area.

Throughout all our restoration work, we’ve observed that the first two years of a new planting are extremely important to get right. Constant maintenance and plant releasing is necessary to get the plants off to a confident start. We are looking to make a date at the beginning of spring where we can all muck in on a sunny day to assist with releasing and enjoy some shared food. A formal invitation will be set up, but please get in touch if you would like to join us.