Monitoring an environmental watering action in Thule Creek to evaluate the contribution of flow via Thule Creek to the productivity of the Wakool River

In September 2019 Forestry Corporation of NSW and the NSW Department of Planning, Industry and the Environment (DPIE) planned two environmental watering actions to deliver water to Thule Creek and the Wakool River from 1) the Koondrook-Perricoota (K-P) Forest via the Thule Regulator, and 2) from Yarraman Channel via the Murray Irrigation Limited (MIL) Thule Escape. The flows from K-P Forest were part of a broader Southern Spring Flow watering action in the Murray River. The environmental flows were expected to flush carbon and nutrients from K-P Forest and Thule Creek into the Wakool River and contribute to the productivity of the Wakool River.

This report describes the results of a monitoring program that examined water quality in Thule Creek and the Wakool River in response to these watering actions. It was hypothesised that flows via Thule Creek to the Wakool River would increase nutrient and carbon concentrations into the Wakool River downstream of Thule Creek relative to the Wakool River upstream of Thule Creek, and that the flows would make a contribution to instream productivity in the reaches of the mid-Wakool River. This project was undertaken by Charles Sturt University (CSU) Institute for Land, Water and Society and was funded by The Living Murray, with in-kind contributions from CSU.

Two watering actions were undertaken in 2019-2020 to deliver environmental water to Thule Creek:
1. A watering action to deliver environmental water from K-P Forest via the Thule Regulator commenced on 8th October 2019 at approximately 15 ML/day and continued until the regulator was closed on 22nd October (14 days). The water arrived at the Thule Regulator on 30th September, approximately 30-35 days after water commenced to flow into the K-P Forest, so the water had been in the forest for approximately 38 to 42 days before being released through the Thule Regulator into Thule Creek.
2. A watering action from Yarraman Channel commenced on 3rd October 2019 and ceased on 8th January 2020 (98 days). This action commenced prior to the release of water from K-P Forest, with the intention to pre-wet Thule Creek and also to serve as a dilution of the expected carbon and nutrient rich water that would be delivered from K-P Forest. The release from the Thule Escape was approximately 36 ML/day (range 30.8 to 39.7 ML/day) and the total volume of water delivered from the Thule Escape was 3,529 ML.

Eleven sites were monitored including seven in Thule Creek, three in the Wakool River, and one in Yarraman Channel. There was a low level road crossing between sites 1 and 2, a small lagoon between sites 3 and 4, a larger lagoon between sites 4 and 5, and a block bank at site 6.
Field sampling was conducted once per week over a period of 15 weeks between mid-October 2019 and the end of January 2020, with the sample weeks grouped into two phases in response to the two environmental watering actions:
• Two consecutive weeks monitoring flows from K-P Forest via Thule Regulator
• Fifteen consecutive weeks monitoring flow from Yarraman Channel via the MIL Thule Escape
Indicators used to assess the responses to the watering actions were:
• Hydrological variables: Mean and range of daily discharge
• Carbon and nutrient parameters: dissolved organic carbon (DOC), total phosphorus (TP), bioavailable phosphorus (FRP), total nitrogen (TN), nitrates + nitrites (NOx), ammonia (NH3), and Chlorophyll-a
• Spot water quality: DO, temperature, electrical conductivity, pH, turbidity
• Field observations of vegetation and biota
Outcomes of water delivered from K-P Forest via the Thule Regulator
Environmental water released from the K-P Forest via the Thule Regulator had increased concentrations of dissolved carbon and nutrients than the source water from the Murray River.
• The concentration of DOC in the water released from the Thule Regulator (20 mg/L) was much higher than the source water from the Murray River. DOC in the Murray River in September 2019 during the Southern Spring Flow watering action increased from 3 mg/L to only 5 mg/L during the peak of the flow pulse. The concentration of DO in the water released from K-P Forest did not go below the 4 mg/L threshold for fish stress.
• Unfortunately the water delivered through the Thule Regulator did not connect with a remnant pool near Lower Thule Road because the bed of Thule Creek was very dry, so the water soaked into the sediment. The downstream flow of the water from K-P Forest was also impeded by a low level road crossing located approximately half way between the Thule Regulator and the Lower Thule Road Bridge. This demonstrates that even small structures can influence the flow in systems with very flat river gradient. Thus the water did not create a pulse of carbon and nutrients into the Wakool River as planned.

Outcomes of water delivered from Yarraman Channel via MIL Thule Escape
Environmental water delivery from Yarraman Channel via the Thule Escape commenced on 3rd October 2019 and ceased on 9th January 2020 (98 days). At the start of the watering action there was a pool in Thule Creek near the Thule Escape but two lagoons downstream of the escape were dry.
• Similar to the delivery of water from K-P Forest, the water travelled very slowly downstream and soaked in to the creek bed along the way, thus taking many weeks to fill the two lagoons further downstream in Thule Creek. The progress of the water downstream to the Wakool River was obstructed by a block bank in the creek at site 6 that held up the water for several weeks but was later breached to allow the flow to connect with the Wakool River.
• The DOC concentration in the water released from Yarraman Channel was approximately 4-5 mg/L. After the water had travelled downstream and inundated shallow areas of small red gum trees, grass, leaf litter and bare soil, the DOC concentration increased to 10-12 mg/L at site 4 and was as high as 80 mg/L at site 6 in water that was standing for several weeks behind the block bank. Concentrations of DOC at site 6 (block bank) were similar to results of a glasshouse experiment simulating the leaching of carbon in standing water on a floodplain. After the block bank was breached and the water started to flow downstream the concentration of DOC in the water reduced to between 20 and 30 mg/L at sites 4 to 6.
• Despite the low DO concentrations, field staff and a landholder on Thule Creek did not observe any dead fish in Thule Creek at any time during the study and fish were observed moving about. Only the sites that had DOC greater than 20 mg/L had DO concentrations below 4 mg/L (threshold associated with fish stress). Dissolved oxygen concentrations below 2 mg/L (threshold associated with fish deaths) were recorded at sites 5 and 6 and where water was held up behind the block bank.
• The carbon rich water was clear (low turbidity) and there was a large amount of invertebrate activity and improvement in abundance and condition of trees and emergent and submerged aquatic plants. Birds, including pelicans, were observed feeding in Thule Creek and there were over 50 colonial waterbird nests observed by DPIE staff, including darters nesting at the upper lagoon and little black cormorants nesting in thick red gum regrowth between the two lagoons.
The flow in Thule Creek connected with the Wakool River between 19th and 26th Dec 2019. A DOC pulse was detected in the Wakool River on 2nd January where the DOC concentration at Greenhill Lane (10.1 mg/L) and Bookit Island Lane (8.3 mg/L) was higher than that in the Wakool River (6.6 mg/L) upstream of the confluence with Thule Creek. The pulse of DOC was short lived. By 16th January there was no difference in DOC concentration between the upstream and downstream Wakool River sites because flows from Thule Creek had ceased. These results suggest that delivery of environmental water down Thule Creek can leach significant sources of carbon and nutrients from soil, vegetation and leaves and contribute to the instream productivity of the Wakool River at times when there is connectivity of flows.
In summary, the Thule Escape watering action created a small blackwater pulse that had positive and no detectable negative outcomes. The small pulse increased hydrological connectivity and improved vegetation condition including fringing vegetation and emergent/submerged aquatic plants in Thule Creek. Despite there being some sections of the creek that had low dissolved oxygen levels, no fish deaths were observed. The pulse provided opportunities for movement, reproduction and recruitment of invertebrates, frogs, native fish and birds.
Recommendations
• Future flow actions need to consider the impacts of the low level road crossing (between sites 1 and 2) and block bank at site 6 on water delivery in Thule Creek.
• For future flow actions when parts of Thule Creek have a dry starting condition, the discharge from K-P Forest through the Thule Regulator should be increased to ensure flow is sufficient to cover losses into the sediment as well as continue flow downstream.
• There are opportunities for the delivery of water through K-P Forest to create small productive pulses in the Wakool River. To avoid risks of hypoxic blackwater it is important to maintain water flow and prevent banking up or water standing for long periods of time when carbon loads can increase and have detrimental effects on DO concentration.
• Even though this watering action occurred during extremely hot weather conditions there were no fish kills observed. As only a short length of the creek experienced high DOC and low DO concentrations, fish had the opportunity to move upstream or downstream to areas where there was water with higher concentration of dissolved oxygen. This suggests that when delivering water to create pulses of carbon to improve productivity it is essential to maintain connectivity with other parts of the system to allow fish to move if they need to do so.
• The increased capacity of Thule Escape after it has been upgraded will provide increased options to manage the flows and dilution of carbon. When the Thule Escape upgrade is completed a series of flow trials should be undertaken under different conditions to improve our understanding of how to balance the flows from K-P Forest Regulator and the Thule Escape with flows in the Wakool River to achieve the best outcomes for the river ecosystem.
• The relationship between the various components that influence ecosystem metabolism is complex and the scale and timing of flows is important. The magnitude of flows through K-P Forest will need to be optimised and the time of year of water delivery is critical to avoid poor water quality outcomes, due to the strong influence of temperature on river processes.