Multi-thread planform diversity originates from competition between migration and floodplain development on mid-channel bars

Longstanding efforts to explain the origin of river planforms are typically based on a simplified dichotomy (or spectrum) between single-thread meandering planforms and multi-thread braided planforms. However, multi-thread planforms in nature are exceptionally diverse. They can be classified as braided, wandering, anastomosing, and more based on various metrics, with definitions across studies that are inconsistent, non-exclusive, and lacking a physical mechanism. To tackle this knowledge gap we investigated four decades of planform dynamics along 49 rivers around the world imaged by NASA Landsat, leveraging established proxies for floodplain development (e.g., NDVI) and a state-of-the-art automated migration-mapping technique based on particle image velocimetry (PIV). Results reveal multi-thread planform diversity originates from competition between migration and floodplain development on mid-channel bars, which we quantify in terms of a floodplain development timescale (T_fp) and a bar turnover timescale (T_bar). If bars migrate slowly relative to the pace of floodplain development (T_bar >> T_fp), bars are converted to floodplains by fine-sediment deposition and vegetation growth, resulting in an anastomosing planform. In contrast, if bars migrate quickly  (T_bar << T_fp), they remain bars because migration reworks vegetation and fines before any floodplain develops (braided). And if bars migrate at intermediate speeds (T_bar ~ T_fp), they form a mosaic of partially converted bars and floodplains (wandering). Put to practice, these findings can advance our ability to predict future planform changes in response to climate change and human activities; and help decipher ancient planforms left behind in the sedimentary record.