What is it?

Silt is a fine sediment that can be found suspended in water or at the bottom of a water body forming soil. Physical (e.g frost shattering) and chemical (e.g rain) weathering break down surrounding rocks that then wash into our rivers and lakes.

Cromwell receives most of its sediment from the Kawarau river which in turn gets it from various rivers (e.g. Dart and Shotover), streams and waterfalls upstream. One study found the Shotover river carries between 1.6 and 2 million m³ of silt annually. Before the dam, the sediment would be carried down the Clutha river out to the pacific ocean 75km southwest of Dunedin.

Why is it an issue?

For over 30 years the Clyde Dam has prevented the natural transport of sediment from the Kawarau and Clutha rivers to the ocean. Reports created when the dam was being built concluded that sediment build-up was expected in the Kawarau Arm over time. The issue however is the poor management of its transition. 

The Kawarau arm is no longer viable for swimming, fishing or boats due to silt deposits. The sediment build-up has resulted in the shift of the shoreline, loss of the jetty and shallowing of the boat ramp. The visual beauty of Old Cromwell is further impacted by the driftwood and Lagarosiphon that wash onto the sediment.

Silty sediment is ideal for the aquatic weed Lagarosiphon major to establish. Efforts to control Lagarosiphon are regularly hampered by suspended sediment in the water that deactivates herbicides and hessian sacks, used to prevent weeds from rooting to the lakebed, are quickly covered.

Sediment is typically high in organic matter which removes oxygen from the water as it decomposes. Lower oxygen levels result in changes to the pH and increased pollutant levels meaning native ecosystems collapse. Aquatic plant species are vital for the survival of native invertebrates, fish and birds dependent on them for food and shelter. This lack of oxygen levels also creates foul odours in the warmer month, further impacting recreation.

​Tackling the issue

Contact Energy is legally responsible for managing the visual effects caused by the Silt in the Kawarau arm. The Lakebed and shoreline of the Kawarau arm is Crown land managed by LINZ (Land Information New Zealand). 

​Ways to solve/manage the issue

The sediment arriving in Cromwell is a natural process that we cannot prevent useless if we build another dam. This however would only move the problem upstream and cost a substantial amount. The issue we face is how to manage the sedimentation and the transition of the Kawarau Arm. 

Accelerating sedimentation and dredging are current methods being researched. Building up the sediment already settled will create a land that can be planted and landscaped for recreational use. Dredging the sediment and moving it elsewhere would allow access to the boat ramps and swim areas lost. Both come with benefits and problems.

​What will be done

Contact Energy has monitored the area since 1993 but has not implemented any significant management other than compliance with consent conditions to remove silt at the Bannockburn Inlet and Lowburn Inlet. LDCT has recently sought the legal opinion of a specialized RMA barrister to support the legal opinion that Contact Energy is responsible for the visual effects caused by the Silt in the Kawarau arm. This opinion was supported by the ORC.

New proposals are expensive and complicated but Contact Energy has a legal responsibility to manage the visual amenity values. LDCT is assisting them to research new methods that will support the transition and improve the visual amenity in Old Cromwell.

Once the Kawarau Arm has transitioned back into a river, sedimentation will no longer occur. This is because the faster water flow will deposit it further into the lake.

References

[1] NZGS (2017) Dynamic Characteristics of the Lake Wakatipu Silty Sediments. To view Click here
[2] ORC (2006) Learning to Live with Flooding: A Flood Management Strategy for the Communities of Lakes Wakatipu and Wanaka. To view Click here
[3]NIWA (2016) Ten Year Management Plan for Lagarosiphon at Lake Dunstan: 2016 to 2025. To view Click here
[4] ORC (2011) Clutha river/Mata-au Plan. To view Click here
​[5] NIWA (2012) Guidelines for Artificial Lakes. To view Click here

Main image: Unsplashed
Images in order: LDCT
​​Bringing together the needs of the community (Clutha/Mata-Au) and the activity of stakeholders (Kawarau) to achieve a healthy and happy environment for people and wildlife (the junction).
Charity Reg: CC57320