Geomorphic and vegetative river recovery in a small coastal catchment of New South Wales, Australia: implications for flow hydrology and river management

Throughout eastern Australia, and much of the New World, river systems have undergone a series of hydro-geomorphic and vegetative changes following colonisation. Recent research has shown that river recovery is occurring, as demonstrated by improved geomorphic and vegetative condition, but there is limited information on the extent of returning riparian vegetation and its effect on roughness and flow hydrology. Using the Allyn River, New South Wales, Australia, as a case study, historical aerial photographs and satellite imagery were used to characterize the structural changes brought about by geomorphic recovery and quantify the amount of riparian vegetation cover that has returned to the system since the 1940s. Along the Allyn River, total riparian vegetation canopy cover has increased from 30% in 1940 to 38% in 1967 and 63% in 2016. Using space-for-time substitution and retrospective analysis, riparian vegetation roughness, calculated using a terrestrial laser scanner (TLS) gap fraction method, showed an increase in average Manning's n from 0.0156 in 1940 to 0.0194 in 1967 to 0.0372 in 2016 for flows that reach a 2 m stage height (approximately 1 in 2 yr flood average recurrence interval). As a result, flood wave travel time has extended by over 8 h (or 220%) for small to moderate within-channel flows, but further river recovery is required to affect major flow hydrology. This study highlights the outcomes of catchment-scale passive river recovery and provides improved methods for quantifying site and stage height specific vegetative roughness. The demonstrated influence of increased vegetation cover on flow hydrology and geomorphic condition holds significant promise for agencies, practitioners and land managers to achieve substantial river recovery outcomes using active and passive vegetation rehabilitation and river management.