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.