Provisionally accepted

Given the impacts and costs of ongoing river degradation and extreme rainfall and flooding caused by climate change, improving and maintaining river health is becoming increasingly urgent to meet global sustainable development goals. Scaling up and working with nature-based solutions at regional, catchment and river corridor scales is critical. One way to achieve this is to work within a recovery-led approach to river restoration, incorporating processes of geomorphological and vegetative recovery to build river corridors in areas where they have been fragmented. Geomorphologically-informed cost-benefit analyses have not previously been conducted at either the riverine or riparian scales at the required catchment or regional scales. We applied Marxan in a novel way to conduct the first cost-benefit analysis for geomorphologically-informed restoration of river systems. We estimated that restoring 75,500km of rivers in the NSW coastal catchments will cost $8.2 billion, which compares favourably with recent insured losses from individual floods and projected future losses. We developed Marxan scenarios based on three general approaches to river management: ‘ad hoc and reactive’, ‘working with restoration’ and ‘corridors’, for current and future time periods. We found that there are significant current and future financial and non-financial benefits, as well as lower initial total and per hectare costs for restoration, by fully focusing on the ‘working with restoration’ or ‘corridors’ approaches and moving away from the ‘ad hoc and reactive’ approaches that dominate current practice. Implementing targeted restorations based on a continuous sequence of ‘corridor’ scenarios over time provides optimal holistic solutions to improve geomorphic condition and increase restoration potential at the landscape level. Our study demonstrates the use of Marxan as an accessible tool to manage the complexity of prioritisation and to implement and cost landscape-level restoration scenarios over time. Our study also demonstrates the positive off-site feedbacks that occur through multiplier effects as restorations occur and corridors are built. Geomorphologically informed decisions become more robust, transparent, cost-effective, consistent across catchments and adaptable to local circumstances and evolving river management priorities.