ELSEVIER Geomorphology 14 ( 1995) 235-253 River channel adjustment to meander cutoffs on the River Bollin and River Dane, northwest England J.M. Hooke University of Portsmouth, Department of Geography, Buckingham Building, Lion Terrace, Portmouth, PO1 3HE, UK Received 24 October 1994; revised 26 March 1995; accepted 8 May 1995 Abstract Morphological adjustment of a channel after cutoff should be expected, but rarely has the mode and timescale of adjustment in natural channels been measured. Four natural cutoffs which have occurred since 1980 on two rivers in northwest England have been monitored from pre-cutoff stage. Two of the cutoffs were neck cutoffs of large loops; two were chute cutoffs in which significant bend curvature remained. All the cutoffs were the terminal result of progressive bank erosion and/or scour of the floodplain rather than overflow and all the cutoffs took place in peak flow events, though two of these were of relatively low magnitude-high frequency. Rapid widening and sedimentation took place within the straight channels produced by neck cutoffs, immediately after breaching. Adjustment was mainly by formation of multiple riffles and bars, producing a variable morphology in the first 24 years. Subsequently, the morphology became more regular and in one case, had stabilised within about eight years. Progressive steepening and acceleration of bank erosion appears to have been propagated upstream in one case, but in the other cases erosion was very localised. Rates of vertical accretion at the entrances to the old channel were much higher than most quoted in the literature. Rates of change in all morphological, sedimentary and biotic components exhibit an exponential decline with time but with slightly differing timescales. On these active, gravel-bed streams major adjustment within the new channels may be completed within 6-12 years though lakes persist in the abandoned channels for much longer. 1. Introduction Cutoffs and ox-bow lakes are some of the classic features of meandering rivers and are recognised as fundamental components of the floodplain of mobile rivers. These features have been studied for both their geomorphological and engineering importance, using a range of empirical and theoretical approaches. Vari- ous techniques have been employed to investigate their morphology, stratigraphy and distribution including historical mapping, remote sensing, coring and sedi- mentological analysrs. Long term development has been traced and various phases have been identified and modelled. The ra.tes and patterns of sedimentation have received particular attention because of their importance in interpretation of the alluvial record. Very few observations of cutoffs actually taking place and of subsequent adjustments of the channel have been made, with notable exceptions, e.g. Johnson and Payn- ter, 1967; Mosley, 1975a. Information on the nature of channel changes within cutoff reaches and on the times- tale of adjustment are needed to help predict and under- stand impacts of cutoffs and to interpret past evidence of cutoffs. Opportunity to measure and document these aspects of meander cutoffs has occurred from monitor- ing channel change on two rivers in northwest England. Based on the evidence of these case studies, the aims of this paper are: 1. to describe the processes and mechanisms of cutoff, 2. to analyze the morphological adjustments to cutoff, 0169-555X/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDfO169-555X(95)00110-7