Evaluating the significance of event and post-event sediment dynamics in a first order tributary using multiple sediment budgets.

Investigations of sediment transfer in upland catchments are rarely conducted over a sustained period of time, consequently a full understanding of the changing nature of sediment supply, storage and yield is often lacking. Three recent sediment budget studies from the Wet Swine Gill headwater catchment in the Lake District, Northern England, UK (a 0.65 km2, first-order tributary), provide evidence of changes in sediment transfer dynamics over the period 2002–2008. The first sediment budget in 2002 describes the impact of a hillslope debris slide and channelised debris flow event, where the former was the dominant budget component. The termination of the debris flow in the Wet Swine Gill channel meant that the vast majority of slide failure material was not transferred to the downstream fluvial system. However, subsequent modification of exposed hillslope sediment by post-event erosion processes and gully development resulted in ongoing erosion. A second sediment budget (June 2003–January 2004) demonstrated sediment yield downstream of the in-channel debris slide deposits far exceeds upstream fluvial sediment delivery by two orders of magnitude (c. 4,000 kg and c. 20 kg, respectively). Erosion of sediment from the exposed hillslope failure scar (c. 1300 kg) was less than channel erosion (c. 3300 kg), and sediment transfers from both the hillslope and channel sediment sources are sensitive to high-magnitude, low-frequency trigger events including summer thunderstorms, and winter rainfall/ snow-melt events. However, linear regression analysis only demonstrates weak or insignificant relations between meteorological conditions and sediment yield. A final sediment budget in April 2008 shows the significance of both hillslope (inclusive of gullying) and channel erosion/ transfer processes over the six-year monitoring period. In this budget, like the first sediment budget, the hillslope system is marginally more dominant, and therefore demonstrates a further switch in the relative significance of hillslope and channel system components. When interpreting such findings the potential uncertainty in the budget components, particularly in the unmeasured residual components, should be considered, as the magnitude of the error can be large. These results suggest that contemporary event and post-event sediment flux in small headwater catchments are more complex than short-term investigations would initially suggest. Furthermore there is a clear need for continued, longer-term monitoring of sediment system dynamics and associated hydro-meteorological conditions, in order to develop understanding of how future climate change may impact upland sediment systems