A reach, catchment and multiple catchment scale assessment of the patterns and controls of historic upland river planform adjustments

Abstract

The supply, transfer and deposition of sediment from channel headwaters to lowland valleys and lakes, along the upland sediment cascade, is a fundamental process in upland catchment geomorphology. The continuity of the sediment cascade coupled with local geomorphic controls can be partly understood by quantifying river planform adjustments both in space and over time. However, few studies have adopted rigorous quantitative assessments of sediment continuity and planform adjustment beyond the reach scale over historical time periods or considered key controls governing the stability of upland river channels (e.g. climate, anthropogenic activity). This research presents an assessment of the patterns and geomorphic variables of upland river planform adjustment and stability over the past 150 years. A nested sampling strategy is adopted exploring sediment continuity and planform adjustment at the reach, catchment and multiple catchment (regional) scales in the Lake District upland region, north-west England. In total, 270 rivers and streams (total length: 597 km) were studied across 17 catchments in the upland region (total area: 1250 km2) for six dates from 1860s – 2010. Reach scale investigations focused on exploring the impact of the extreme Storm Desmond (December 2015) flood event on St John’s Beck in the Bassenthwaite catchment.

A total of 29,832 stable and adjusting reaches were mapped in the Lake District upland region. Over the full period of analysis (1860s – 2010), 21 % (128 km) of rivers and streams studied were classified as adjusting. Regionally, the highest percentage of river and stream lengths mapped as adjusting between 1860s – 2010 were observed in the Ennerdale (37 %), Wasdale (32 %) and Calder (29 %) catchments (Western Lake District). These catchments showed persistent adjustment and active zones of sedimentation in high order channels over the last 150 years. This is attributed to a high supply of sediment to the fluvial system, greater accommodation space for lateral planform adjustment, and <20 % of the rivers and streams were anthropogenically modified. In contrast, the Kent, Haweswater and Sleddale (Eastern Lake District) showed persistent planform stability and <11 % of the river and stream lengths were mapped as adjusting between 1860s – 2010. This is attributed to narrow topographically confined channels, high specific stream powers and high sediment continuity. In the Kent catchment >57 % of the river and stream lengths were anthropogenically modified via reinforced banks and flood embankments restricting planform adjustment.

At the reach scale, the influence of a low frequency, high magnitude flood event (Storm Desmond) on river planform adjustment was quantified. However, with increasing spatial and temporal scale the correlation between high magnitude flood events and planform adjustments are harder to define. Anthropogenic activity (e.g. channel engineering, or mining) had a significant influence on river planform adjustment and stability at the reach scale. Regional patterns of geology and the legacy of glacial processes help condition sediment supply, channel slope, and valley bottom width (confinement) thereby setting the general environmental template in which channels adjust in the Lake District. Valley bottom width was found to be an important variable determining the accommodation space for lateral planform adjustment and sedimentation. Planform adjustments occurred in reaches with a mean valley bottom width of 120 ± 190 m.

This research has demonstrated the importance of considering planform stability in a sediment continuity framework across all scales of the stream and river network. The methodology developed provides a quantitative assessment of planform adjustment patterns and geomorphic controls, which aids understanding of historic river behaviour and provides context for current and future river management and restoration strategies.