Abstract
The supply, transfer and deposition of sediment from channel headwaters to lowland sinks, is a fundamental process governing upland catchment geomorphology, and can begin to be understood by quantifying 2D river planform adjustments over time. This paper presents a catchment scale methodology to quantify historic patterns of 2D channel planform adjustment and considers geomorphic controls on 2D river stability. The methodology is applied to 18 rivers (total length = 24 km) in the upland headwaters of the previously glaciated Wasdale catchment (45 km2), Lake District, northwest England. Planform adjustments were mapped from historic maps and air photographs over six contiguous time windows covering the last 150 yr. A total of 1048 adjustment and stable reaches were mapped. Over the full period of analysis (1860–2010) 32% (8 km) of the channels studied were adjusting. Contrasts were identified between the geomorphic characteristics (slope, catchment area, unit specific stream power, channel width and valley bottom width) of adjusting and stable reaches. The majority of adjustments mapped were observed in third and fourth order channels in the floodplain valley transfer zone, where the channels were laterally unconfined (mean valley bottom widths of 230 ± 180 m), with low sediment continuity. In contrast, lower order channels were typically confined (mean valley bottom widths of 31 ± 43 m) and showed relative 2D lateral stability. Hence, valley bottom width was found to be important in determining the available space for rivers to adjust. Over the full period of analysis 38% of planform adjustments involved combined processes, for example, as bar and bend adjustments. The study demonstrates the importance of stream network hierarchy in determining spatial patterns of historic planform adjustments at the catchment scale. The methodology developed provides a quantitative assessment of planform adjustment patterns and geomorphic controls, which is needed to support the prioritisation of future river management and restoration.
Highlights
Systematic assessment of the spatial and temporal patterns of river planform adjustments provides important insights for understanding current and potential future river behaviour (Hooke and Redmond, 1989a; Winterbottom, 2000; Brierley and Fryirs, 2005a; Lisenby and Fryirs, 2016; Rinaldi et al, 2016)
The stream orders differ in length, steepness, confinement and specific stream power reflecting the longitudinal variation in the upland headwater channels (Fig. 5)
Mean valley bottom width increases by a factor of 18 downstream from 31 ± 43 m in second order channels to 550 ± 30 m in fifth order channels (Fig. 5)
Summary
Systematic assessment of the spatial and temporal patterns of river planform adjustments provides important insights for understanding current and potential future river behaviour (Hooke and Redmond, 1989a; Winterbottom, 2000; Brierley and Fryirs, 2005a; Lisenby and Fryirs, 2016; Rinaldi et al, 2016). This is because channels adjust through erosion, transfer and deposition of sediment (Lewin, 1977; Thorne, 1997) and channel planform adjustments reflect sediment continuity.
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