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

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 the 1840s. This study examines subsequent changes to the morphology, sediments and vegetation of the Brisbane River in relation to the major drivers of channel change. Documentary evidence from pioneers, paintings, newspapers,maps, surveys, photographs, and instrumental flow records suggests that within 20 years of the introduction of sheep grazing, compaction and degradation of catchment soils and surface drainage produced a shift from perennial to seasonally ephemeral flow and channel incision in minor tributary valleys. The main channel remained stable until the 1850s in the Estuary, and until the 1890s in the middle reaches, where bank erosion increased average channel widths by 18%. Compared to rivers in temperate areas, the Brisbane River has been relatively resilient to changes in land use and land cover. Rates of lateral channel migration have been low since at least 1885, and the level of the channel bed has been stable since 1894. It is shown that the present-day compound channel is a pre-European form with dimensions adjusted to floods with decadal return periods. Increases in sediment supply associated with the incision of tributary streams and later, from widening of the main channel, is consistent with regional evidence for the predominance of channel erosion. This implies an ongoing channel adjustment to changes associated with European land use change.