Simulating post-LGM riverine fluxes to the coastal zone: The Waipaoa River System, New Zealand Phaedra Upton a,n , Albert J. Kettner b , Basil Gomez c , Alan R. Orpin d , Nicola Litchfield a , Michael J. Page a a GNS Science, Lower Hutt, New Zealand b CSDMS, INSTAAR, University of Colorado Boulder, CO, USA c Department of Geography, University of Hawai’i at M ¯ anoa, HI, USA d NIWA, Wellington, New Zealand article info Article history: Received 28 February 2011 Received in revised form 31 January 2012 Accepted 1 February 2012 Keywords: Simulating sediment flux Holocene Last Glacial Maximum Waipaoa River System abstract HydroTrend, a climate-driven hydrologic transport model, is used to simulate the suspended sediment discharge of the Waipaoa River System (WRS) over the last 5.5 kyr. We constrain the precipitation input with a paleo-rainfall index derived from the high-resolution Lake Tutira storm sediment record. The simulation is extended to 22 ka using a lower resolution version of the model, constrained by terrestrial and marine paleoenvironment indicators and a simulated model of northeast New Zealand’s climate at the Last Glacial Maximum (LGM). Comparison of the 5.5 kyr simulation with the shelf sediment core MD97-2122 suggests that the sediment flux variations observed on the shelf primarily reflect changes in rainfall associated with wetter and drier periods of centuries to millennia duration. Storage of sediment on the Waipaoa River floodplain (Poverty Bay Flats) moderates the signal by reducing the sediment flux reaching the coast. During the LGM conditions were more erosive than the Holocene with tussock and grass dominated vegetation. For erodibility four times the Holocene’s and half today’s, the LGM Waipaoa River System would have generated approximately half the current sediment yield and about 3 times the amount generated when the catchment was fully forested. & 2012 Elsevier Ltd. All rights reserved. 1. Introduction Steepland rivers draining tectonically active continental mar- gins are known to be particularly sensitive to environmental change, driving variations in the sediment flux to the ocean over time scales of centuries to millennia (e.g., Dadson et al., 2005, 2003; Gomez et al., 2004; Syvitski et al., 2005). Unlike their longer and larger counterparts that drain passive margins through low gradient and extensive muddy deltas, these short and steep high- yield rivers typically discharge directly to the ocean (e.g., Milliman and Syvitski, 1992). Thus, the terrestrial sediment sources and marine sinks are closely coupled. In steepland river basins the dispersal of erosion products is also amplified by the severity and flashy behaviour of floods, short river courses, and narrow continental shelves (e.g. Dadson et al., 2005; Gomez et al., 2007a; Sommerfield and Nittrouer, 1999). The Waipaoa River System (WRS) in northeastern New Zealand is a prototypical, non-glaciated steepland fluvial system in which the balance of environmental drivers has been the subject of detailed investigation (Gomez et al., 2007a; Kettner et al., 2007). It forms the terrestrial component of the Waipaoa Sedimentary System (WSS) (Fig. 1), a focus site of the MARGINS Source-to-Sink initiative (Carter et al., 2010). As such the WSS is a well characterised and ideal location to quantitatively test the application of sediment models and their ability to hindcast changing catchment yields in response to environmental drivers. Here, we used HydroTrend (Kettner and Syvitski, 2008a), a climate driven hydrological model, along with best estimates of climate variation through time to interpret environmental factors such as vegetation cover and unravel the important drivers of sediment delivery from the Waipaoa River mouth since the LGM. Using a sediment core from Lake Tutira to reconstruct the trend in regional rainfall (Orpin et al., 2010; Page et al., 1994, 2010) and a reconstruction of shoreline progradation across the Poverty Bay Flats (Wolinsky et al., 2010) we extend the 3000 year model of Kettner et al. (2007) back to the mid Holocene (5.5 cal ka, note that all ages used in this manuscript are cali- brated). We also use terrestrial and marine paleoenvironmental indicators in conjunction with a simulated model of northeast New Zealand’s climate at the LGM (Drost et al., 2007) to tune a lower resolution version of the model and extend the simulation back to 22 ka, at the Last Glacial Maximum (LGM). Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/cageo Computers & Geosciences 0098-3004/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.cageo.2012.02.001 n Corresponding author. Tel./fax: þ64 4 570 4600. E-mail addresses: p.upton@gns.cri.nz (P. Upton), kettner@colorado.edu (A.J. Kettner), basilg@hawaii.edu (B. Gomez), a.orpin@niwa.cri.nz (A.R. Orpin), n.litchfield@gns.cri.nz (N. Litchfield), m.page@gns.cri.nz (M.J. Page). Please cite this article as: Upton, P., et al., Simulating post-LGM riverine fluxes to the coastal zone: The Waipaoa River System, New Zealand. Computers & Geosciences (2012), doi:10.1016/j.cageo.2012.02.001 Computers & Geosciences ] (]]]]) ]]]–]]]