Article written by Dr Jack Coats-Marnane, Principal Scientist, Healthy Land & Water.

Forests close to rivers and creeks, known as the riparian zone, play a critical role in maintaining freshwater ecosystem health and water quality. We recently investigated their role in influencing water quality in South East Queensland.

The catchments and rivers of South East Queensland support freshwater biodiversity and provide drinking water to an ever-growing population. Forests close to rivers and creeks are known as riparian zones. They play a critical role in maintaining freshwater ecosystem health and water quality.

As interest in ESG (Environmental, Social, and Governance) motivated or market-based investments grow, accurately predicting and quantifying the benefits of riparian conservation and rehabilitation is becoming increasingly important. Ongoing ecosystem health monitoring and focused research are enhancing our understanding of the value provided by riparian ecosystems.

Recently, we investigated the role of forests in influencing water quality in SEQ, using data collected as part of the South East Queensland Catchment Loads Monitoring Program. The program has been routinely measuring the sediments and nutrients transported by rivers for over 20 years.

Monitoring stations across catchments with different sizes and land-use characteristics allow for long-term trend analysis and comparison in the mass of sediment and nutrients transported within individual catchments and between catchments. Advances in remote sensing have also improved the accuracy of riparian forest mapping in the region.

Our investigation was driven by two critical questions:

Question 1: Have there been any significant changes in sediment and nutrient loads within individual catchments?

This work analysed trends in sediment and nutrient transport within individual catchments to identify significant changes in loads over time and explore potential drivers.

Only two sites showed a change over the monitoring period and sediment loads declined by 2.6 times for Laidley Creek at Warrego Highway, and total phosphorus increased by 1.6 times in Coochin Creek.

The lack of any significant change at most sites was not surprising, given that an analysis of land use indicates there has been very little change in the eight catchments monitored over these periods.

The decrease in suspended sediment loads in the Laidley catchment is likely due to an improvement in river channel condition; however, more investigation is required to determine with more certainty that this is the reason.

Question 2: What is the role of riparian forests in influencing sediment and nutrient loads of catchments?

This work tested the theory that sediment and nutrient loads from catchments decrease proportionally with the increasing proportion of stream-length draining areas of woody vegetation.

This work found that there are significant relationships between the level of woody vegetation cover within the riparian area and the load of sediment and nutrients exported by catchments.
We found that the sediment yield per unit area from a catchment containing no riparian forest is predicted to be between 5 and 10 times higher than that of a fully vegetated channel network (conceptual diagram 1). For total phosphorus (TP) this is between 12 and 21, and for total nitrogen (TN) between 2.4 and 3.9.
Riparian vegetation slows flood waters, reducing erosive power and also acts to stabilise channel banks and areas susceptible to erosion. This explains fine sediment and associated nutrient export from catchments with more riparian vegetation.

So how does this information help inform catchment management?
Improving the water quality of rivers, estuaries and coastal areas protects aquatic ecosystem health and improves water security in the region. This study tells us that conserving existing riparian forests while facilitating their recovery where they have been lost, is key to water quality improvement in the region.

We can also now use this general model (conceptual diagram 1) to predict the water quality benefit of riparian rehabilitation for specific catchments. For example, in the Warrill Creek catchment 37% of the riparian area is forested, with an annual sediment load of 40,143 tonnes per year. Increasing the riparian forest cover from 37% to 65% is predicted to decrease the annual load by ~50% (to 22,500 tonnes per year); increasing to 100% cover would drop this to 11,300 tonnes per year.

This also highlights the scale of work that is required to achieve a meaningful improvement in water quality in catchments of South East Queensland.

The conservation and restoration of these areas is being led by a range of community groups, organisations and landholders across the region. One example is the Healthy Catchments program which has been delivering riparian vegetation works in focal areas, including works in the Warrill Catchment for over a decade. We are confident that if this type of work is scaled up, we will see tangible improvements in water quality across the region.

Conceptual diagram 1. The relationship between catchment suspended sediment loads and catchment riparian forest cover.

Acknowledgement
This research was funded by the Ecosystem Health Monitoring Program partnership a partnership between State and Local Governments of South East Queensland, Water Utilities and Healthy Land & Water. For more information on the program please visit the Report Card website. Technical Reports can be made available through a request to This email address is being protected from spambots. You need JavaScript enabled to view it..

Reference Olley, J., Coates-Marnane, J., Orr, D., Heeley, B., Tsoi, I. 2024. Temporal changes in catchment sediment and nutrient loads and the influence of riparian forests. Technical Report.