In 2011, discharge volumes in the Fitzroy and Proserpine rivers were the largest ever recorded.26 In the Herbert River, the volume was equal to the biggest ever recorded, while the Burdekin River experienced the third biggest.26 Increased freshwater inflow to the Region during flood events carries with it pulses of nutrients, sediments, pesticides and other pollutants including marine debris, which have significant effects on inshore Great Barrier Reef habitats and species.27 Depending on the geology and soil permeability, freshwater also enters estuaries and the sea as groundwater.28 Some mangroves, saltmarsh plants and seagrasses depend on freshwater seepage.28 Some marine animals, for example sea snakes, consume freshwater from submarine groundwater seepages.29 Freshwater also seeps through the ocean floor from drowned river channels called ‘wonky holes’.30 Wonky holes are considered important natural sources of nutrients for coral reefs31 and seagrass meadows32. Changes to terrestrial habitats and infrastructure associated with development in the catchment are affecting the flow of water to the Region (see Section 6.4).
Improved land management is beginning to reduce sediment input.
Sedimentation — the inflow, dispersion, resuspension and consolidation of sediments — has been a natural phenomenon in the Region since the current sea level was reached about 6500 years ago.34,35,36 However, exposure of the Great Barrier Reef to terrestrial sediments and resuspended marine sediments has increased since European settlement of the adjacent catchment.35,37,38 It is estimated that suspended sediment loads are now more than twice as high as before European settlement in the 1850s.39,40 These increased loads affect sedimentation processes. Modelling of pre-European exposure to suspended sediment suggests that its effects were concentrated very close to the coast around river mouths, with the largest plume adjacent to the Burdekin River.34 Modelling for the years 2007 to 2011 indicates a vastly increased area of exposure (Figure 3.4). Inshore areas continue to be exposed to the most sediment, especially areas close to river mouths.37,38 However, during flood events, suspended sediment may be carried long distances — as far as 100 kilometres northward for the Burdekin River plume in the 2010–11 wet season41. Longshore drift41,42,43,44,45,46,47, tides and currents4,48,49,50,51 widely redistribute sediment along the coast and across the continental shelf.49,52,53,54,55 Possible increases in wind speed in the Region20 are likely to cause more sediment resuspension in shallow water.21 Significant investments in land management practices from 2009 to 2013 have resulted in a modelled 11 per cent reduction in the average annual suspended sediment load delivered to the Great Barrier Reef.56 However, there is likely to be a significant lag time before there are measurable and ecologically significant water quality improvements in the Region, with effects continuing for at least decades.57 Activities within the Region that contribute to increased sedimentation and resuspension plumes include anchoring and vessel wash from shipping, dredging, and disposal of dredge material. Recent modelling suggests resuspended sediment could potentially travel considerably further than previously understood.58,59
Figure 3.4 exposure to suspended sediments, 2007–2011
The assessment classes (high, moderate and low) are relative and derived from a combination of scaled river load data and flood plume frequency analysis from remote sensing data. The mean of the five annual distributions was selected as a way of factoring in inter-annual variability in river discharge, although it is recognised that this period was characterised by several extreme rainfall events. Source: Brodie et al. 2013 60