Sand to Mud

Intensive farming, mining and forestry has turned our estuaries from sand to mud and losing productive soil.

Since Parliament’s Select Committee reported back on the Thames-Coromandel District Councils Mangrove Management Bill there has been no public reaction from the Council.  After being all gung-ho about their Bill the strange silence from the Council is probably because the Select Committee gutted the Bill by limiting its cover to only mangroves in the Whangamata harbour and recommended stringent other controls on mangrove removal. 

In the meantime a recent article highlighted immense damage being done to the Firth of Thames and other industries by soil erosion.  NIWA scientist Andrew Swales has been studying the mangrove forest in the Firth of Thames for years and has some disturbing things to say about the ongoing sedimentation.

“New Zealand’s estuaries were once sandy, forest-lined gems. In the Firth of Thames, farmers used to go land yachting on sand flats in the 1940s and 1950s. But since the 1960s, the area has been eaten away by a growing a 1100 hectare mangrove forest. 

the erosion which washes huge quantities of fertile sediment into the country’s waterways is also leaving the land increasingly less productive.

Another piece of key research, underway since late 2016, is the Niwa-led Managing Mud project. 

Niwa scientist Dr Andrew Swales, who heads Managing Mud, said fine sediment reduced light levels in rivers and coastal waters and smothered sea grass and animals living in the seabed.

“Basically since widespread catchment deforestation occurred about 150 years ago, we’ve seen a jump in soil erosion by about 10-fold,”

The result was the formation of intertidal mudflats in place of sandflats that had characterised many estuaries before catchments were deforested, Swales said.

An article co-written by Swales said some small, shallow systems, such as Hot Water Beach and Waikawau, on the eastern Coromandel Peninsula, would fill with sediment and die.

Writing by early settlers gave some idea of the way many estuaries used to be. “A lot of these historical accounts talk about going for picnics on sandy beaches. These days they’re buried under a couple of metres of mud,” Swales said.

“We’re losing the productivity of our soils, and we’re causing this major environmental problem in our rivers and estuaries. It’s a double whammy.”

An example is the Firth of Thames. Long-established farmers in the area had told him they used to go land yachting on sand flats in the southern firth in the 1940s and 1950s. Since the 1960s a muddy mangrove forest of about 1100 hectares had developed in the area. Research by Swales and others found mangrove forest has colonised an 800-metre wide, 11sq km area.

The Waihou River is estimated to deliver an estimated 160,000 tonnes a year of suspended sediments into the firth each year, with another 30,000 tonnes from the Piako River. Altogether, about 70sq km of intertidal mudflats, up to 5km wide, have built up in the southern firth.”

Now that we have estuaries in this state the answer is not the knee jerk reaction of TCDC to remove the mangroves – they are proven to be a highly effective barrier to storm surges which will increase with sea level rise.  Instead, we must make sure the most effective tree planting efforts are made on our most erosion-prone slopes, and other methods are used to reduce erosion.  In many places that will not be pinus radiata.

But as  Swales points out its a balancing act because some ongoing sedimentation is required to keep the mangroves alive and healthy and be able to combat sea level rise .