Exceptional river gorge formation from unexceptional floods

Area of Science:

• Geomorphology
• River dynamics
• Landscape evolution

Background:

• Understanding bedrock river incision and knickpoint retreat is crucial for predicting landscape responses to environmental changes.
• Sparse field data currently limits our knowledge of these geomorphic processes.

Purpose of the Study:

• To investigate the rates and mechanisms of rapid gorge formation and bedrock erosion.
• To analyze the controlling factors of erosion in a newly formed bedrock channel.

Main Methods:

• Utilized eyewitness accounts and quantitative surveys of a rock-cut dam spillway experiencing overtopping floods.
• Analyzed erosion rates, including headward retreat, incision, and widening over a six-year period.

Main Results:

• Observed rapid amphitheatre-headed gorge formation in unweathered granite.
• Erosion rates showed no correlation with flood magnitude or bedload availability.
• Bedrock structural patterns (faults and joints) were identified as the primary drivers of erosion, primarily through fluvial plucking and block toppling.

Conclusions:

• Extremely rapid transient bedrock erosion is possible even in mechanically strong rocks and moderate flood discharges.
• Bedrock discontinuities significantly control landscape change and facilitate rapid gorge formation and knickpoint retreat.
• Findings challenge existing models by highlighting the dominant role of structural geology in rapid bedrock erosion.