What lies beneath: Revisiting braided river morphodynamics with topobathymetric lidar

The assessment of four-dimensional channel change through topographic differencing has evolved rapidly from a research frontier to an established tool to inform sediment and flood hazard management. This transition has been driven largely by the growing availability of broad-area, multitemporal lidar surveys that enable system-scale analysis of river adjustment across space and time.

Despite these advances, the application of lidar-based differencing in fluvial environments remains fundamentally constrained by the limited availability of bathymetric data to complement conventional infrared lidar terrain models. This data deficit is commonly managed through the prescription of spatially distributed elevation uncertainty models, or more crudely by assuming that compensating patterns of erosion and deposition across exposed channel areas are representative of areally averaged changes in mean bed level. Such assumptions are frequently applied in large braided rivers, where much of the channel bed is exposed at low flow and morphodynamics are dominated by lateral channel mobility.

These assumptions can rarely be tested quantitatively due to the scarcity of repeat, high-quality bathymetric datasets. Here, we address this limitation by comparing system-wide patterns of channel adjustment for a large piedmont braided river derived from conventional hydroflattened DEMs and from seamless topobathymetric lidar elevation models. The analysis uses three surveys of the Rees River, New Zealand, acquired between 2021 and 2025 using co-mounted Riegl VUX-240 (1550 nm) and VQ-840-G (532 nm) lidar systems on a helicopter platform.

Our results reveal a five-fold increase in net volumetric change when derived from topobathymetric terrain models compared to hydroflattened DEMs. This difference reflects substantial sedimentation within wetted channels between 2022 and 2025, a process that is poorly captured by hydroflattened models. Moreover, aggradation within wetted anabranches evident in the 2022 topobathymetric DEM is not balanced by subsequent channel incision by 2025. The net effect is a reversal in the inferred direction of bed-level change: topobathymetric analysis indicates a significant increase in mean bed elevation, whereas hydroflattened analyses imply marginal degradation.

These findings highlight the importance of sediment fluxes within the wetted network of braided channels and demonstrate that morphodynamic interpretations based on exposed-bed differencing alone may be fundamentally misleading without effective bathymetric correction.