What to do with our boggy wetlands?

What to do with our boggy wetlands?

Sunday, November 1, 2015

It is well known that run-off sediment, phosphorus (P) and nitrogen (N) from farm land can be effectively reduced using existing or constructed wetlands. The Invermay deer farm is sited on low-altitude hill country, and there are several wetland areas already existing along stream drainage sites. We were keen to find out how we could use these naturally occurring sites to assist in reducing sediment loss off the farm.

We initially focussed on an existing wetland site located at one of the waterway exit points on the deer farm boundary. This catchment drains about 40 ha of the deer farm through a series of dry gullies through which water flows only when it rains. This then flows into a permanent flowing stream that drains into and out of two ponds (probably originally constructed for duck shooting).

We were a little unsure what to do with this site, so we invited James Sukias from the National Institute of Water and Atmospheric Research (NIWA) to Invermay to discuss how to manage our wetlands. James is well-known for his knowledge in this area and he guided us through the principles of developing and maintaining wetlands to reduce sediment loads in waterways.

I will admit that we had visions of having to get in some heavy machinery to dig waterway channels, spending loads of money on new fencing and planting massive amounts of woody vegetation along riparian margins. But the truth was, the existing wetland was almost ideal in its present form. It had almost all of the characteristics needed to do the job we want

  • Sedimentation ponds
  • A slow meandering water course
  • Abundant appropriate vegetation (reeds and rushes) that trapped sediments and shaded the water
  • A wetland area of about 2.5 ha that was more than adequate to cope with water run-off from the 40 ha catchment

Wetland area on the Invermay Deer Farm PDF

All that was required was removal of deer grazing from the upper reaches of the wetland (with sheep permitted during dry seasons) and complete removal of all livestock from the lower wetland area (which required a minor upgrade of sheep fences as it is already outside the deer farm).

While we do plan to plant some native trees and shrubs around the margins, this is largely for aesthetics, as they will probably provide little added functionality to the wetland.

The lesson in this for us was that wetland management is not onerous, and simple steps to enhance existing wetlands are often sufficient. However, we do acknowledge that the effectiveness of the sedimentation ponds and wetlands will depend upon continued maintenance and management.