Temporal variations in fluvial processes on an active meandering river over a 20-year period

Abstract

An active meandering river in NW England has been monitored annually for more than 20 years. Data on the occurrence of erosion, deposition, calibre of sediment and morphological changes have been recorded for a sequence of > 50 bends in a 10 km length reach. The data on process activity have been analysed for patterns and variation over the period 1981–2002. Occurrence of floods is hypothesised to be the major control on activity and this paper aims to test that relationship and to identify whether any major phases of activity emerge at this timescale. Data on magnitude of annual peak discharges and on the frequency of peaks over a threshold show a phase of relatively low peaks in the early 1980s, a phase of variable discharge and some low flows from 1987–1997, and a phase of high peaks 1998–2002. Frequency of high flows has increased through the period. Analysis of the relationship of processes to discharge characteristics indicates that erosion takes place on a high proportion of bends every year and that coarse material (cobbles) is deposited on bars in most years. Process thresholds for bank erosion and coarse sediment transport and deposition are below bankfull and occur on average several times a year. A trend of severity of erosion activity to winter peak discharge during the 1980's did not continue into the period of high flows of the late 1990's but deposition was similar in both periods. It is hypothesised that vegetation may be influencing response. Vegetation growth on the bars increased from the late 1980's and is likely to have decreased erosion but enhanced deposition. Number of frost days has decreased over the period but other factors could also be influencing vegetation. The occurrence of the process activity is compared to overall changes in morphology on the river measured from photogrammetric mapping of the course in 1984, 1996 and 2001. These show differences in average channel width but the interannual variability of processes raises questions of the numbers and sequences of flood events needed for different phases and hydromorphological responses of the channel to be identifiable.