Original paper

Influence of hillslope-to-channel and tributary-junction coupling on channel morphology and sediments: Bowderdale Beck, Howgill Fells, NW England

Farraj, Asma A.L.; Harvey, Adrian


The influence of hillslope-to-channel and tributary-junction coupling on stream channel morphology and sediment is examined on two study reaches of Bowderdale Beck, an upland stream in the Howgill Fells, northwest England. The two reaches differ in their coup ling characteristics. The upper study reach has several major tributary-junction fans and several large erosional scars cut into the valley sides, resulting in a series of major discrete sediment supply points along the reach. The lower reach has no major fans and only small valley-side scars, resulting in only a few minor dispersed sediment supply points along the reach. The channel is dominantly a single-thread gravel-bed stream. Channel widths, measured at 96 sites in the upper reach and 58 in the lower reach, show a very weak downstream increase, but with local wider occasionally braided reaches downstream of major sediment supply points, especially in the upper reach. At 51 sites (27 upper reach: 14 lower reach: plus 10 source area sites) along the 5 km length of channel, scaled photographs of 0.5 × 0.5 m quadrats of the exposed bar surfaces were taken in the field. Measurements of clast size, angularity, and sorting were carried out from the photographs. The results exhibit dominance by streamside sediment sources with local trends in sediment size and shape characteristics away from sediment sources but very little overall downstream trend. The currently active sediments are compared with those deposited by a 100-year flood which occurred in 1982 (19 sites). Those sediments are coarser but show similar patterns in relation to sediment supply points. Differences between the two reaches are interpreted to relate to the different coupling characteristics and to stream power.


hillslope-channel couplingchannel morphologysedimentation zonesdownstream sediment trendsstream powerhowgill fells