Awatere River’s Cloudy Water: 100-Million-Year-Old Rocks to Blame

Marlborough River Clouded by Ancient Rocks: A 100-Million-Year History

The Awatere River in Marlborough, New Zealand, is experiencing persistently cloudy water due to sediment originating from 100-million-year-old rocks formed during the time of the supercontinent Gondwana. The river has consistently failed to meet water clarity standards set by both the Marlborough Environment Plan and the National Policy Statement for Freshwater Management, prompting investigation by Marlborough District Council officials.

The issue isn’t a recent one, and according to Dr. Martin Crundwell, an honorary academic at the University of Auckland and teaching fellow at Victoria University of Wellington, there’s limited practical recourse for significantly improving the river’s clarity. His investigation, conducted with Marlborough District Council senior environmental scientist Steffi Henkel, points to the region’s geological history as the primary driver of the turbidity.

Gondwana’s Legacy in Marlborough’s Bedrock

The source of the sediment lies in the greywacke bedrock of the Pahau Terrane, formed during the early Cretaceous period. As Crundwell explained to Marlborough councillors, this bedrock originated from a process of tectonic subduction where the Pacific Plate slid beneath the Gondwanan Plate. This action scraped sediments from the seafloor, piling them up along the edge of Gondwana – the landmass that would eventually fragment into present-day continents like New Zealand.

“This is where Marlborough sits… it’s caught by these rocks known as the Pahau Terrane, which is the youngest rocks of the tallest terrane,” Crundwell stated. Today, the Pahau Terrane comprises up to 70% of the rock within the Awatere catchment, particularly in the Inland Kaikōura ranges and the northern Southern Alps. The greywacke is particularly susceptible to erosion, contributing significantly to the sediment load in the river.

The Mechanics of Erosion

The erosion process is multifaceted. Crundwell highlighted that more than 15% of the bedrock is comprised of fine grain released through freeze-thaw weathering. This occurs when water within the rock freezes and expands, gradually breaking it apart over repeated cycles. This fine grain is then washed into streams like the Tone River, which feed into the Awatere.

Erosion around Mt Tapuae-o-Uenuku, Marlborough’s highest mountain, also contributes to the river’s turbidity. The mountain itself is believed to be between 66 and 100 million years old, and its volcanic rock adds to the sediment load. 1News reported on these findings.

The Awatere Faultline’s Role

Adding to the complexity, the Awatere faultline plays a significant role. The rocks in the fault zone are “crushed” and “pulverized” through tectonic forces, creating even finer sediment that washes into streams in the eastern Awatere catchment, such as the Hodder River. This process, known as tectonic deformation, further exacerbates the turbidity issue.

Limited Options for Improvement

Given the widespread nature of the problematic bedrock, Crundwell concluded that substantial improvements to the Awatere’s clarity are “neither feasible nor practical.” While reforestation in the headwaters of the catchment could offer some benefit, it wouldn’t be enough to meet the environmental expectations outlined in the Marlborough Environment Plan. The situation is not unique to the Awatere River; the Clarence River is similarly affected by the same geological conditions.

Council Response and Future Outlook

Marlborough District Council officials acknowledge the challenges. Councillor Barbara Faulls thanked Crundwell for his presentation, noting its educational value. The council intends to continue supporting landowners through existing programmes like Catchment Care and Hill Country Erosion to mitigate the issue where possible.

Rural representative Simon Harvey emphasized the ongoing collaboration between farmers and the council on these mitigation efforts. Councillor Gerald Hope sought to understand the river’s condition historically, learning from Crundwell that while vegetation cover has changed since European settlement, the fundamental geological conditions contributing to turbidity have remained largely consistent for centuries.

The findings underscore the powerful influence of geological history on environmental conditions, and the limitations faced when attempting to alter natural processes operating over vast timescales. While localized mitigation efforts can aid, a complete reversal of the Awatere River’s turbidity appears unlikely given the underlying geological realities of the Marlborough region.