Suspended sediment transport and the implications on river channel breakdown: northern Macquarie Marshes, NSW

The lower Macquarie River flows northwest from Burrendong Dam to its confluence with the Barwon River and forms part of the Murray – Darling Basin. Large-scale (i.e.climate and topography) controls determine the volume, energy of flow and the suspended sediment transport capacity down catchment. Unlike river systems that increase in size and capacity with distance down catchment towards coastal regions, the Macquarie River flows inland where decreases in capacity and size reflect the semi-arid nature of the climate and the lack of perennial tributaries. This study has identified that the small-scale controls of declining discharge, suspended sediment transport and inchannel vegetation promote channel breakdown in the distal reaches of the fluvial system. The impact of declining discharge and in-channel vegetation blocking on thecontemporary suspended sediment transport regime varies in both space and time in the lateral and longitudinal dimensions. Longitudinal and lateral hydrologic connectivity and suspended sediment transport were assessed using flood recurrence intervals, discharge, and total suspended solids data and channel corridor geomorphic units. Contemporary in-channel and floodplain sedimentation rates, using optically stimulated luminescence and unsupported lead-210, were investigated in the northern Macquarie Marshes. These variables were used as tools to assess the spatiotemporal variability of longitudinal and lateral hydrologic connectivity and suspended sediment transportdown catchment. Surface sediment accretion rates ranged from 2.43 g cm-2 yr-1 upstream of channel breakdown, 0.47 g cm-2 yr-1 within the zone of channel breakdown, and 0.077 g cm-2 yr-1 downstream of channel breakdown. Channel – floodplain hydrologic connectivity increases down catchment. The results show a discontinuity of suspended sediment transport through the northern Macquarie Marshes, promoting a positive feedback cycle between declining discharge and specific stream powers, enhancing in-channel sedimentation and vegetation growth. In-channel vegetation blocking and bed aggradation increases the recurrence of overbank flows and sustains the essential ecosystem services provided by dryland wetland systems.