The Waipaoa River Story: how changing connectivity modulates catchment-scale response times and prospective river futures

The 2208 km² Waipaoa River catchment, the onshore portion of the Waipaoa source-to-sink (S2S) sedimentary system in the East Coast of New Zealand’s North Island, is a tremendously data-rich study area for obtaining insights into catchment function and connectivity (Kuehl et al. 2016). There have been longstanding concerns about erosion severity and mitigation dating back to 1895 (Hill, 1895). High rates of erosion in the Waipaoa have been the defining catchment management issue since the mid-twentieth century, triggered by forest clearance for pastoral agriculture in the late nineteenth and early twentieth century and primed by a combination of highly erodible lithologies, steepland terrain, regular intense storm events, and slopes undercut by postglacial river incision. The magnitude of erosion in the East Coast Region since European forest clearance has exceeded that in any other part of New Zealand.  Connectivity relationships in the Waipaoa catchment have been profoundly altered. Simultaneous sediment inputs from deforested slopes deliver sediment directly into tributary and trunk stream channels, causing significant bed aggradation and channel infilling. Gullies overwhelm receiving streams, forming alluvial fans and represent the single largest source of sediment to the system. Sediment derived from shallow landslides is significant during storm events, but gully, sheet and riverbank erosion have the potential to generate sediment whenever it rains. In the upper catchment, channel widening contributes further sediment, particularly through bank and cliff erosion. The fine-grained nature of sediment supplied by lithologies in the upper catchment accentuates downstream delivery of materials, resulting in channel contraction and floodplain accretion in the lower Waipaoa.  Efforts to mitigate erosion by afforestation using plantation forestry have been successful in some headwater tributaries, with channel degradation beginning, but this process generates additional sediment, which is conveyed downstream. We use a combination of LiDAR and river channel cross-sections to demonstrate contemporary longitudinal patterns of connectivity in the Waipaoa River and assess the prospective river futures of this anthropogenically altered system.

 

References

Hill, H. (1895) Denudation as a factor of Geological Time. Transactions and Proceedings of the New Zealand Institute, 1895, Volume XXVIII.

Kuehl, S. A., Alexander, C. R., Blair, N. E., Harris, C. K., Marsaglia, K. M., Ogston, A. S., Orpin, A. R., Roering, J. J., Bever, A. J. & Bilderback, E. L. 2016. A source-to-sink perspective of the Waipaoa River margin. Earth-Science Reviews, 153, 301-334.