Quantifying trajectories of geomorphic river recovery through analysis of assemblages of geomorphic units: Outcomes of nature-based river management post severe riparian clearance  

Abstract

The removal of riparian vegetation and instream wood, as well as channelisation, river regulation and sediment extraction, has led to significant adjustments and metamorphosis in many rivers of southeastern Australia throughout the 19th and 20th century, post colonisation. With improvements in river management practice that saw a transition from an engineering approach to a passive nature-based river rehabilitation approach, coincident with a period of minimal flooding, signs of geomorphic and vegetative recovery have been detected since the 1980s across coastal rivers of NSW. Analysis of decadal trends in woody and non-woody vegetation coverage in 20 catchments in the coastal region, has shown on average, a 40% increase in vegetation coverage along riparian corridors since the 1950s. Some catchments now have a current day coverage of between 60% and 80%. Coevally with vegetative recovery has been an improvement in the physical structure and function of these rivers, expressed as geomorphic river recovery. We have used ergodic reasoning to quantitatively analyse changes in the assemblage of geomorphic units that occur for rivers at different stages of geomorphic recovery. To track geomorphic river recovery we developed a semi-automating methodology that integrates Geomorphon tool and supervised classification, to map geomorphic units using Open Access LiDAR and Sentinel-2 images. We analyse the assemblages of geomorphic units for 78 river sections that span eight river types (River Styles), three valley settings and two bed material textures – sand and gravel. We find that geomorphic river recovery is not always linear and occurs in different patterns for different river types. As recovery progresses, adjustments tend to occur at the sub-unit scale by changing the form of individual units. For example, river recovery can be detected by changes in indicator geomorphic units. The presence of benches and islands indicates that recovery is underway across most river types. A statistically significant increase in abundance and area of benches and pools, and a decrease in abundance and area of floodplain steps can also be used to indicate that recovery is underway. Additionally, as recovery occurs, we observe that bank-attached bars tend to become more compound in structure. At the reach scale, confined and most laterally unconfined rivers exhibit linear and non-linear increases in richness, abundance, evenness, and diversity of geomorphic units during recovery. Partly confined rivers show more variable trends for these measures, and channelised fill rivers show decreased diversity. Across this region, river resilience has been tested by severe fire and catastrophic flooding between 2019 – 2022. Remarkably, these rivers have shown high levels of resilience to these extreme events. For example, flood peak travel times have slowed dramatically since the 1980s, attesting to significant increases in geomorphic and vegetative roughness brought about by river recovery. River recovery in these systems is a decadal process. Understanding recovery trajectories post colonisation land clearance has significant implications for the prediction of future channel evolution and provides invaluable insight for nature-based and recovery-enhancement approaches to river management.