Coupling between hillslopes and channels in upland fluvial systems: implications for landscape sensitivity, illustrated from the Howgill Fells, northwest England

The sensitivity of upland fluvial systems depends on the magnitude and frequency of sediment and flood producing events, modified by the internal coupling characteristics of the system. This paper assesses the role of hillslope/channel coupling for the sensitivity of upland geomorphic systems, using evidence from a 30-year monitoring programme of geomorphic change in the Carlingill valley, Howgill Fells, northwest England. In the hillslope zone, there is little sediment supply to the stream system. Locally, slope failures occur in response to extreme events (six such events in 30 years within the ca. 6 km 2 Carlingill valley). In the footslope coupling zone, basally induced gullies are major sediment sources to the stream. Sediment-production events occur ca. 30 times per year, feeding sediment to basal debris cones. Stream floods which can entrain these sediments occur once every ca. 2–5 years. Stream channel morphology is adjusted to this regime. Downstream of gullies, channels are wide and braided; elsewhere they are narrow and single-thread. As the gullies develop, however, this coupling weakens and the eroding slopes eventually stabilise by revegetation. Over the 30-year monitoring period, there has been a progressive trend towards gully stabilisation, and an associated reduction in channel instability. Massive destabilisation may occur in response to rare extreme flood events. Such an event (return period >100 years) occurred in neighbouring Langdale and Bowderdale valleys in June 1982. That event destabilised the system, causing slope failures, fan deposition, and in some places, a switch in channel style to wide braided channels. Since 1982 there has been a progressive recovery by slope stabilisation and single-thread sinuous channels have become reestablished. A different style of extreme event occurred in Carlingill in October 1998, in response to the wettest week in the 30-year period. A slope failure fed debris flows 400 m downslope, almost coupling with the channel system. Future climatic change could render the system prone to destabilisation, through either extreme flood events or through major slope failures.