Donald Creek Catchment Study 2020-2027

Establish monitoring infrastructure in the control and treatment catchments.

Weirs were constructed and infrastructure was set up at weirs A and C, and baseline monitoring for flow and turbidity commenced.

Weirs A and C suffered bank erosion from a significant storm event in November 2020, causing holes in the bank at each site. These sites were repaired.

Baseline sediment load quantified, and baseline stream health documented.

Harvest planning and road lining in preparation for 2021 harvest.

Field trip for local stakeholders.

Roadline harvesting and earthworks on the roads and landings within treatment area 1 commenced in April 2021.

Stakeholder field trip held in May 2021, involving the project partners presenting to council reps, MPI, forest industry reps, Fish and Game. Iwi were unable to attend.

Stream health monitoring sites established above weir A (Plum Creek) and weir C (Cat Creek) within the study catchments, and control sites on the Tadmor River below Cat Creek confluence and Plum Creek confluence.

Baseline data from all four monitoring sites was collected and the first ecological assessment report showed the cover of fine sediment varies between seasons and were generally low; and relatively good habitat and cover for instream fauna present at all sites; and macroinvertebrates and fish fauna indicated relatively healthy streams.

At end-May there was a significant daily rainfall event totalling 65mm for the 24-hour period.  It was the first significant rainfall event since both weir sites became fully operational and was the highest daily rainfall total to date.  This was also the second year under La Nina.

Whenever planning an outdoor event, plan for the worst! As a result of high winds, the stakeholder field trip was moved to the local Tapawera hotel.

Progress report data for harvest period in treatment catchment

Earthworks and harvesting in treatment area 1 commenced.

Characterisation of erosion processes and sediment sources in the cutover and during storms undertaken.

Continued sediment and stream monitoring data collection.

Despite an excessive wet 12 months, harvesting within the treatment area 1 was completed early in this milestone. Earthworks were completed during this period.

Significant rain events in July, September 2021 and February 2022 provided high flow and turbidity data required for the project to ascertain effects of earthworks and harvesting erosion sediment controls (ESC) in treatment area 1.

The ESCs installed throughout treatment area 1 (except for three landings due to contractors still harvesting in these areas/utilising the landings for processing) have been inventoried. The final ESCs will be captured in July/August 2022 when harvesting is finished.

The media campaign between OFO and MPI raised high interest nationally and locally with the project, including a TV segment on Rural Delivery.

Treatment area 1 progress report and Infrastructure Install

Established monitoring infrastructure at Weir B.

Harvesting completed in the treatment area 1 above Weir A.

Catchment ground cover, earthworks rehabilitation and sediment sources monitored for recovery.

Stream health surveys after harvest activities conducted and results were analysed in relation to suspended sediment load from both the control and treatment catchment including consideration of climate variables.

Two field trips held with over 80 attendees. Project partners attended and presented on their respective subject matter.

Quarterly water surveys by Cawthron indicated water quality at all sites is generally high. Nutrients and faecal indicator bacterial concentrations are generally low.

Considering the paired forested stream catchments in the Donald Creek forestry block, the only indication that tree harvesting in Treatment Area 1 (Plum Creek) was affecting water quality was spot temperatures are higher than those recorded on previous years’ sampling and relative to the Control Catchment (Cat Creek) temperatures in the December 2022 and February 2023 water quality results.

In the post-harvested treatment area 1, it was found the shading of the stream had decreased and there was an increase in streambank erosion. There was also a general decrease in instream habitat quality for macroinvertebrates and fish.

To date, there has been no quantitative or anecdotal evidence to suggest the harvesting of Treatment Area 1 has affected the freshwater ecology in the Tadmor River (below the Plum Creek sampling site). This is one of the two control monitoring sites outside the study area.

As a result of a storm event in July 2022, Weir A burst and had extensive repairs. Weir C also required repairs. Weir B was built to a different specification in discussion with Tasman District Council principal hydrologist.

The science partners expanded to include sediment expert Dr Murray Hicks, working with Envirolink on sediment loads and turbidity.

Treatment Area 1 progress report and monitoring of the three weir sites.

During this milestone period mechanical land preparation and replanting was completed.

Catchment ground cover, earthworks rehabilitation and sediment sources were monitored for recovery.

Stream health surveys were conducted, and results were analysed in relation to suspended sediment load from both the control and treatment catchment. This included consideration of climate variables.

Previously existing erosion sediment control (ESC) measures have been maintained, with a few new measures installed. Small scale runoff and sediment mobilisation has been observed along roads and on landings but little of this appears to be reaching Plum Creek (in Treatment Area 1). Many of the sediment traps are effective in capturing mobilised fine sediment.

Gravel accumulated at Weir pond A at Plum Creek through 2023, so the pond needed to be excavated to enable the instrumentation to operate reliably. Surveys were done to quantify the volumes of gravel excavated and deposited. A resurvey of Plum Creek main channel found evidence of reactivation of many previously mapped bank erosion sites and a small number of new sites, suggesting the source of gravel was bank erosion.

The only indication that forest harvesting in Treatment Area 1 (Plum Creek) was affecting baseflow water quality was the elevated nitrate-nitrogen concentrations and higher spot temperatures compared with observations pre-harvest and in the unharvested Control Catchment (Cat Creek). In the post-harvested Treatment Area 1, it was found the shading of the stream had decreased. There was evidence of an increase in streambank erosion, general decrease in instream habitat quality for macroinvertebrates and fish and decreased microbial activity. While quantitative streambed suspended sediment results from Autumn 2024 suggested an increase in the fine sediment content of the streambed material, the increase in suspendible sediment has not persisted.

To date, there has been no quantitative or anecdotal evidence to suggest the harvesting of Treatment Area 1 has affected the freshwater ecology in the Tadmor River (below the Plum Creek sampling site). This is one of the two control monitoring sites outside the study area.

While not part of the original project scope, water clarity sampling commenced in February 2024 and will continue monthly through to the end of the project (30/6/2027).

The suspended sediment yield from Treatment Area 1 (Plum Creek) has continued to exceed that from the control catchment (Cat Creek) by a significant margin.