RIVER CHANNEL CUTOFF DYNAMICS, SACRAMENTO RIVER, CALIFORNIA, USA

We measured patterns of river channel migration and cutoff between 1904 and 1997 on a 160 km meandering alluvial reach of the Sacramento River by intersecting a sequential set of river channel centrelines mapped from a field survey and aerial photography. We identified approximate dates and locations of cutoffs and quantified cutoff dimensions. Twenty-seven chute and 11 partial cutoffs occurred over this 93-year time interval, with an average of one cutoff approximately every 2.5 years or 0.0029 cutoffs per kilometre per year. The average rate of lateral channel change was over the study period was 5.5 +/- 0.6 m year(-1) (approximately 0.02 channel widths per year) due to progressive migration and cutoff combined. An average of 5% of the total channel length moved laterally via chute cutoff at a rate of 22.1 +/- 3.3 m year(-1) versus 94% of channel length that moved via progressive migration at a rate of 4.7 +/- 0.5 m year(-1). The remaining 1% of channel length migrated via partial cutoff at a rate of 13.0 +/- 2.8 m year(-1). Although channel cutoff was less predominant mode of channel change than progressive migration in terms of channel length, an average of 20% of the total floodplain area change between successive centrelines was attributable to cutoffs. Peak cutoff frequency was concentrated temporally between 1964 and 1987 and was also spatially clustered in specific active sub-reaches along the valley axis over the entire study period. We hypothesize that the probability of channel cutoff is a function of both channel geometry and discharge. Bends that experienced chute cutoff displayed an average sinuosity of 1.97 +/- 0.1, an average radius of curvature of 2.1 +/- 0.2 channel widths and an average entrance angle of 111 +/- 7 degrees, as opposed to average values for bends migrating progressively of 1.31 +/- 0.01, 2.8 +/- 0.1 and 66 +/- 1 degrees, respectively. The sinuosity of Sacramento River bends experiencing chute cutoff appears to have been consistently declining from 2.25 +/- 0.35 channel widths in 1904 to 1.54 +/- 0.23 channel widths in 1987. We hypothesize that this trend may be due in part to the influence of land-use changes, such as the conversion of riparian forest to agriculture, on the 'erodibility' of bank and floodplain materials. For the post-dam flow regime (1937 on), cutoff frequency was significantly correlated with an estimate of cumulative overbank flow. Copyright (C) 2010 John Wiley & Sons, Ltd.