Using Drainage Area Power-Law Relationships to Remotely Test for River Capture

Fluvial systems are sensitive indicators of a region's tectonic and climatic history, responding to changes by adjusting their erosional and depositional regimes, or by altering their flow direction to mimic large scale changes in topographic slope and aspect. At times these adjustments result in reorganization events, such as river capture or reversal. Geomorphological features commonly cited as evidence for drainage reorganization include anomalous drainage patterns, barbed tributaries, downstream narrowing of river valleys, and windgaps. However, the identification of river capture and reversal events through landscape indicators alone can present a significant challenge, due to the difficulty of identifying landforms uniquely attributed to reorganization events and the lack of preservation in rapidly changing environments, necessitating a quantitative method for the remote analysis of drainage reorganization. Empirical studies predict that river systems in steady-state exhibit a power-law scaling between drainage area and channel length, where drainage area increases consistently and predictably along a stream's length. We postulate that fluvial systems that have experienced river capture should deviate from this predicted relationship: A captured drainage adds an anomalously large drainage area over a short channel length, which should manifest as an abrupt 'jump' in the linear relationship of drainage area - channel length in a log-log plot. We tested the predicted drainage area to channel length relationship for several locations with documented field evidence for river capture events, including the Sorbas basin in Spain and the eastern Tibetan Plateau. The resulting data were compared with data from the Driva basin in southwestern Norway, where geomorphological evidence exists for drainage reorganization, but conclusive, datable markers such as fluvial terraces formed by paleo-drainages are rare, as most have been destroyed by subsequent glaciation. Abrupt jumps in drainage area at the confluence of some tributaries to the Driva river's trunk stream correspond with barbed tributaries, windgaps, and other geomorphological evidence of river capture, providing a promising example of this method's application.