Abstract

Can localized deposition resulting from river avulsion produce downstream sediment-starvation effects similar to those in channels downstream from dams? We explore this question by examining a 130 km reach of the Saskatchewan River (Canada) downstream of an active, 500 km2 avulsion belt initiated in the 1870s. Progradation history and measurements of bedload and channel-bed grain size indicate that little coarse sediment has escaped the avulsion belt since its inception. Comparisons of 17 channel cross sections surveyed in 1911, 1954–1956, and 2011–2012 show that bankfull areas enlarged by deepening and widening in the upper two-thirds of the study reach since 1911. Between 1911 and 1954–1956, average channel area increased by 35% in the upper 45 km, and as much as 44% locally. From 1954–1956 to 2011–2012, additional enlargement occurred in the upper 35 km, averaging 52% since 1911. These increases occurred despite a 24% decrease in mean annual discharge since 1913. These changes are interpreted as effects of sediment sequestration in the upstream avulsion belt. Net erosion by sediment-deficient outflows from areas of avulsive deposition may be an underappreciated process in evolving floodplains.

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