Four Years of Meander‐Bend Evolution Captured by Drone‐Based Lidar Reveals Lack of Width Maintenance on the White River, Indiana, USA.

Meandering rivers experience fluctuations in width whenever riverbanks migrate in different directions or at different rates, which can be observed after individual floods. However, meandering rivers maintain approximately constant widths over decadal timescales. This implies some timescale below which width fluctuates as banks migrate independently, and above which width is maintained by a bank‐coupling process. This coupling is thought to occur either as point bar deposition events induce cutbank erosion (bar‐push), or as cutbank erosion events induce point bar deposition (bank‐pull). This coupling, however, has been challenging to observe in natural rivers due to limited event‐scale field data. We present results from a 4.5‐year campaign with 22 drone‐based lidar surveys of a single point bar and cutbank (∼0.35 km2 in area) on the White River near Worthington, Indiana, USA. The middle point bar experienced net erosion (5,400 m3), but net aggradation (17,100 m3) between 2019 and 2022 when including perennially submerged regions. This aggradation was less than the 35,700 m3 of cutbank erosion over the same period. Combined, we have observed widening (1.58 m/yr bend‐averaged; 3.08 m/yr near apex) over the study period as point bar deposition has not kept up with cutbank erosion. Finally, we suggest that the difference between bar‐push and bank‐pull as width‐maintenance mechanisms may not be resolvable by observing bend widening or narrowing alone without an advancement of current theory, such as determining a long‐term equilibrium width and measuring deviations relative thereto. Plain Language Summary: Rivers are critical resources for humans, their economies, and the environment. Meandering rivers gradually wander across landscapes as their inner banks grow and their outer banks erode. Our understanding of this growth and erosion is still limited over the timespan of individual floods, which are likely the events that cause river movement. Partly due to a lack of observations, this is a critical knowledge gap that needs to be filled for humans to more‐safely build and live alongside rivers. Over 4.5 years, we collected 22 high‐resolution laser scans of the landscape of a single meandering river bend on the White River near Worthington, Indiana, USA. We tracked growth and erosion by comparing these maps to one another. We found complex behavior occurring due to individual floods, but that year‐over‐year changes are more predictable. While the research community has a long‐held understanding that meandering rivers generally have stable widths over decades (unless water or sediment supplied to the river change), we used historical aerial photographs and modern laser scanning to find that this bend has been widening for >45 years. Key Points: We collected 22 drone‐based lidar scans of a single, rapidly migrating point bar and cutbank over 4.5 yearsWe observed high spatial and temporal variability in both point bar and cutbank deposition and erosionThe point bar did not accrete enough to match cutbank erosion, continuing an apparent ∼55 years of overall meander‐bend widening [ABSTRACT FROM AUTHOR]