River channel adjustment to meander cutoffs on the River Bollin and River Dane, NW England

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 2–4 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.