Channel change on the River Dee meanders, 1946–1992, from the analysis of air photographs

Abstract

Gurnell et al. (1994) identified spatial and temporal trends in the planform of the River Dee on the Welsh–English border over a 115 year period (1887–1992) by overlaying information primarily derived from historical maps within a GIS. This paper focuses on channel change within the same river reach over the last 50 years based on the analysis of information from six sets of air photographs within a GIS. The shorter time period of study has resulted in greater difficulty in identifying channel changes which exceed the errors incurred in information extraction from the photographs and registration to a common map base. However, consistency in the spatial and temporal trends identified from different sources and for different stretches within the study reach support interpretation of channel change. Air photographs contain more information that is relevant to the interpretation of processes of channel change than can be obtained from maps. For example, information on bank cover and the pattern of sediment deposition are explored. However, information obtained from air photographs requires careful handling if errors in data extraction and registration are to be minimized. The analysis of channel planform information from air photographs presented in this paper confirms some of the tentative conclusions previously drawn from the analysis of information derived from maps of the same river stretch. Additional information on downstream changes in the spatial extent and pattern of sediment deposition and on temporal trends in bank vegetation cover, that has been extracted from air photographs, provides further details of possible adjustments to flow regulation, indicates the importance of vegetation cover to bank sedimentation processes and illustrates a downstream change in the mode and distribution of sedimentation across the fluvial–tidal transition. © 1997 by John Wiley & Sons, Ltd.