Lisa Harper: Much still to learn about erosion

The Whangaehu River, laden with eroded silt, cut into Andrew Pearce's Kauangaroa farm in June last year. Photo / Bevan Conley

Since the 1950s farmers have been working to control erosion, but there's a lot we still don't know about the processes involved.

Erosion is a gradual wearing away of land by wind, rainfall or wave action. Sometimes it happens dramatically, as we have seen in North Canterbury paddocks after the swarm of earthquakes.

It is a natural process, but can be accelerated by inappropriate management, such as allowing stock to over-graze pasture or taking a bulldozer where it really shouldn't go.

Whether natural or not, the end result can be loss of top soil, flooding and increased levels of phosphorus and sediment in downstream rivers.

Generations of farmers have planted trees, fenced off steep areas and retired their most erodible land in an effort to limit damage and keep any effects on-farm.

These types of measures can help stabilise hillsides and also lower the risk of animals being lost in rough country.

You generally can't just plant trees or put up fences and then walk away. It requires ongoing maintenance.

Trees planted too close together stop grass growing beneath, while trees allowed to get too big may make erosion worse.

Their added weight can cause unstable land to slide, while too big a 'sail' up top catches the wind and topples the plant.

Surprisingly, despite decades of effort, we have much to learn about sediment and erosion processes.

Time delays as sediment moves through the system and the variability of sediment loads are complicating factors. Sometimes scientists find something unexpected.

Planting forestry has in some cases worsened sediment levels in the water downstream, with the change in vegetative cover triggering a re-shaping of the stream bed, in turn increasing sediment in the water over the short to medium term.

It's important to understand the processes, as this informs what management methods are going to be most effective.

Farmers strive to know what works (or doesn't) so they know where best to direct their time and investment, and how long it will take to get results, both on-farm and in the water downstream.

While you can never stop erosion completely, nobody wants to see landslides or valuable soil washed down the river.

- Dr Lisa Harper is a regional policy adviser for Federated Farmers