River channel planform changes in upland Scotland : with specific reference to climate fluctuation and landuse changes over the last 250 years

Abstract

Rates of river channel change in three contrasting Scottish upland environments have been studied within the context of Climatic fluctuation and landuse changes over the last 250 years. The object of the research was to assess the spatial and temporal variation in channel types, the main controls on channel pattern and the dominant modes of channel adjustment. This was undertaken in a hierarchic framework with sites being investigated at three spatial scales. At a macro-scale, the spatial and temporal variation in channel pattern was evaluated through a random sample of river channel segments for each study area, derived from the first and second editions of the 1:10,560 0.S. maps plus the 1:10,000 third edition. Each channel segment was classified within a map-based channel system typology, specifically constructed for upland Scotland. Measures of activity collected for each sample incorporated sinuosity, braiding and lateral shift indices. Flood histories were reconstructed for each study area on the basis of discharge records, long rainfall records and contemporary accounts, to assess if there was any evidence for climatic change, fluctuation or periodicities. Estimates of the recurrence interval of rainfall and runoff events of differing magnitude, frequency and duration were assessed. Data, mainly of a qualitative nature, were derived from contemporary sources and estate plans to evaluate whether any landuse changes could have changed the runoff regime and sediment mobility within each catchment. At a meso-scale, 7 to 9 channel segments (already identified as "active" within the macro-scale study) were subject to a more detailed process-response analysis, using sequential aerial photographs. Finally at a micro-scale, the unit stream powers at these sites were studied in relation to specific runoff rates thereby relating channel process to channel form. The strength of the controls on channel planform type varied in degree with the area studied. The glacial legacy, the positioning of local baselevels and sediment size were found to be dominant controls. In terms of channel dynamics, the position of the Channel planform in relation to process thresholds and the existence of a quasi-equilibrium condition were both very important. In terms of process-response, the following general observations hold true. An extreme event of high RI (>100 years) will have a major disruptive impact if there is room for expansion of the channel system and providing thresholds for sediment transport are exceeded. If these thresholds are high, the fact that the channel has not recently been disrupted may also be important. The modes of expansion across the active area depend on the type of channel involved. Different study areas have different types of Channel pattern present and thus a greater likelihood of certain types of planform adjustment. The role of more moderate events (10-50 years) varies principally with sediment size and Channel slope. Small-scale modification may take place where stream powers associated with more moderate events exceed competence thresholds. It was found that process rates were highly variable in both time and space and that present rates were not necessarily representative of the past 250 years. Even within this timespan, there have been periods of increased activity in response to increased discharges of moderate magnitude (eg. 1870s-1880s within the Dee study area) and random extreme magnitude floods (eg. between 1948-1956 in the Tweed study area). The impact of landuse change, especially in relation to sediment mobilisation (Dee and Spey study areas), and speed of runoff (Tweed study area) also appeared to be important.