River response to European settlement in the subtropical Brisbane River, Australia

Publication date: Available online 24 December 2015
Source:Anthropocene
Author(s): J. Kemp, J. Olley, T. Ellison, J. McMahon
The response of river channels to land cover and land use changes in large areas of the tropics and subtropics is poorly documented. Arable agriculture and grazing was introduced to the subtropical catchment of the Brisbane River, Australia, by European settlers in the1840s. This study examines changes to the morphology, sediments and vegetation of the Brisbane River using documentary evidence from pioneers, newspapers, maps, surveys, photographs, and instrumental flow records. Evidence suggests that 20 years after settlement, a shift from perennial to seasonally ephemeral flow in the major channels was associated with incision and gullying of minor tributary valleys. In the main channel, bank erosion increased average channel width by ∼18%, but rates of lateral channel migration have been low since at least 1885, and the level of the channel bed has been stable since 1894, except in the vicinity of a major reservoir. The timing of bank erosion is closely associated with the removal and progressive degradation of the riparian forest. The present-day compound channel form is a pre-European adaptation to the variable subtropical runoff, with dimensions adjusted to floods with decadal return periods. Compared to rivers in temperate areas, this enlarged channel has proved relatively resilient to changes in flow regime and sediment supply brought about by changes in catchment soils and vegetation. Riparian vegetation appears to be the critical control on channel stability in compound, flood-dominated rivers. Changes in catchment dynamics and seasonal flow regime may have produced more significant effects on the riverine ecology in subtropical Australia than has so far been recognised.



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