Geomorphic Controls on Historical Channel Planform Changes on the Lower Pages River, Hunter Valley, Australia

Abstract

Historical planform changes in a 14.7 km reach of the lower Pages River were determined to assess whether they were autogenic (inherent in the river regime) or allogenic (driven by external changes) in nature so as to better focus river management activities and river restoration works. A pattern metamorphosis or complete change in river morphology occurred during the February 1955 flood. The peak discharge of this event exceeded the slope and grain size (intrinsic) threshold for braiding, converting the narrow, slightly sinuous stream to a wide, braided-like river. Five subsequent intrinsic threshold-exceeding floods did not cause further bar development because an over-widened channel already existed. Autogenic channel planform changes included sinuosity variations due to lateral migration and pattern metamorphosis due to the exceedance of a discharge–slope–grain size geomorphic threshold. Allogenic channel planform changes included: (1) realignment/channel straightening and artificial cutoffs by river training works; (2) lateral migration by increased bank erodibility due to riparian vegetation clearing; (3) lateral migration by the operation of a transitive geomorphic threshold involving the onset of a flood-dominated regime after 1946 and increased catchment runoff after 1830 due to large-scale clearing of catchment vegetation; and (4) the occurrence of a large flood in February 1955. Multiple forcing factors have clearly caused historical channel planform changes of the lower Pages River, making the design of river management and restoration works a complex matter outside the scope of simple formulaic protocols.

Keywords:

• Lateral migration
• pattern metamorphosis
• artificial cutoffs
• river restoration
• intrinsic geomorphic threshold
• transitive threshold

Acknowledgements

For their assistance with various aspects of this work, I thank Peter Wem, the late John Gardiner, Michael Eather, Glen Adamson, Steve Thatcher, Eric Hatfield, the late Darcy Erskine, Christine Erskine, Vanessa Erskine, Aaron Erskine and Pam Steenkamp. Associate Professor Mike Melville and Dr Anita Chalmers constructively criticised a draft manuscript. The support of the former NSW Water Resources Commission is gratefully acknowledged. The journal referees also suggested improvements to the draft.