Fitzroy Sediment Story November 2015 Prepared by Stephen Lewis, Bob Packett, Cameron Dougall, Jon Brodie, Rebecca Bartley and Mark Silburn Fitzroy sediment story. A Report for the Fitzroy Basin Association Report No. 15/74 November 2015 Prepared by Stephen Lewis1, Robert Packett2, Cameron Dougall2, Jon Brodie1, Rebecca Bartley3 and Mark Silburn4 1Centre for Tropical Water & Aquatic Ecosystem Research (TropWATER) James Cook University Townsville Phone : (07) 4781 4262 Email: [email protected] Web: www.jcu.edu.au/tropwater/ 2Department of Natural Resources and Mines, Rockhampton, Queensland, Australia 3CSIRO, Brisbane, Queensland 4068, Australia 4 Department of Natural Resources and Mines, Toowoomba, Queensland, Australia Fitzroy sediment story Contents Executive Summary ................................................................................................................................. 5 1. Introduction .................................................................................................................................... 1 2. Do Fitzroy River discharge and particulate constituents reach vulnerable marine ecosystems, what are the effects (if any) and what parts of the Great Barrier Reef are influenced by both direct exposure and secondary effects? ........................................................................................................... 3 3. How far are the particulate constituents transported in the Great Barrier Reef lagoon, what is their fate and when are they transported? .......................................................................................... 10 4. Where do the particulate constituents come from in the Fitzroy catchment, what are their sources and are there priority areas for management? ....................................................................... 16 5. What are the key erosion processes that release the priority sediment to the rivers and have erosion rates in the Fitzroy catchment changed over time? ................................................................ 22 6. Overall summary and conclusions ................................................................................................ 23 7. References .................................................................................................................................... 25 List of Figures Figure 1. Average annual rainfall map for the Fitzroy River Basin .......................................................... 2 Figure 2. Aerial extent of the 1991 freshwater plume from the Fitzroy River ....................................... 4 Figure 3. Extent of the freshwater plume from the Fitzroy River in 2011 .............................................. 5 Figure 4. Coral luminescence reconstruction for the Fitzroy River ........................................................ 7 Figure 5. The response of photic depth and Fitzroy River discharge in Fitzroy inshore, Fitzroy coastal and Keppel Bay........................................................................................................................................ 9 Figure 6. The response of photic depth to low flow and high Fitzroy River discharge events ............. 10 Figure 7. Total suspended solids (TSS) concentrations measured along the salinity gradient in the 2003 flood plume .................................................................................................................................. 11 Figure 8. The distribution of the mud (<63 µm) and sand (>63 µm) fractions in Keppel Bay .............. 12 Figure 9. The sediment budget for the Fitzroy River <63 µm fraction sediment over the past 100 years ...................................................................................................................................................... 14 Figure 10. Geology of the Fitzroy River catchment area ...................................................................... 20 Figure 11. Land use map of the Fitzroy Basin ....................................................................................... 21 Fitzroy sediment story List of Tables Table 1. Sediment volumes deposited in the floodplain, estuary, beach ridges, sand bars and Keppel Bay for bulk and <63 µm particle size fractions over the past 8000 and 100 years ............................. 15 Table 2. Proportion of the major geological sources in the Fitzroy River estuary, coastal zone and catchment ............................................................................................................................................. 17 Fitzroy sediment story Executive Summary A review of the available information in regards to the origin, transport and impacts of fine sediment and particulate nutrients from the Fitzroy River Basin to the southern Great Barrier Reef (GBR) lagoon was undertaken. It was found that the evidence suggests an increase in water, sediment and nutrient loads from the basin compared to predevelopment conditions. The increase in fine sediment delivery to coastal waters has increased water turbidity during flood plumes and later via resuspension due to wave action and swell. Increases in particulate nutrients such as nitrogen and phosphorus bound to fine sediments also decrease photic depth via chronic eutrophication and biomass in the marine water column after floods. Decreased photic depth has negative effects on coral fringing reefs and seagrass meadows in Keppel Bay and further afield during large flood events. It is the largest flood plumes that have the most widespread impact on reef and seagrass systems. From a reef health perspective, the priority should be targeted management to reduce sediment erosion from catchments that contribute the majority of large flood plumes. Sediment tracing and load monitoring studies have found that basaltic lands used for cropping are a major source of fine sediment and nutrients from a concentration (and per hectare yield) perspective compared to lands used for grazing. Broad‐scale cropping occurs on large areas in the Theresa Creek, Nogoa and Comet rivers sub‐catchments and to a lesser degree (area‐wise) in the Callide Creek and Dawson River sub‐catchments. Cropping also occurs on the floodplains of most streams where black soil alluvium is found throughout the Fitzroy Basin. Continued and improved best management practice is considered a high priority for these intensively cropped basalt areas to reduce fine sediment and nutrient transport to the southern GBR lagoon. It was found that the Connors River sub‐catchment contributes a high number of large floods on a long‐term annual average basis. This catchment also produces a reliable base flow of relatively high quality water for downstream water users such as the centres of Rockhampton and the Capricorn Coast. Maintaining or improving ground cover in the Connors sub‐catchment is considered a priority management action in regards to future sediment and nutrient transport to Keppel Bay. Any changes in land use that involve intensive agriculture such as cropping could result in an increase in sediment and nutrient loads to the coast from this sub‐catchment. Improved management of grazing lands to reduce sediment supply from gully and scald erosion is considered a priority in the Fitzroy Basin. Sediment tracing studies in the Burdekin Basin have shown that subsoils appear to be the major component of fine sediment found in stream monitoring studies. Evidence of severe gullying and scalds can be found in the grazing lands of the Fitzroy Basin and preliminary data from a study underway suggests that cattle ramps cut into the banks of lower order streams may be contributing a larger volume of fine sediment to waterways than previously thought. There is an urgent need to quantify the contribution of fine sediment and particulate‐ Fitzroy sediment story bound nutrients from cattle‐induced riparian damage. There is also a need to conduct field research to monitor the sediment that reaches corals and seagrass and trace this sediment back to a specific source in the catchment. Fitzroy sediment story 1. Introduction The Fitzroy River drains the single largest area (approximately 143,000 km2) of the Great Barrier Reef (GBR) catchments and discharges into the largest estuary of GBR (~60 km in length) and then into Keppel Bay (Figure 1). The Keppel Islands within the bay host a series of inshore fringing coral reefs, which are periodically exposed to discharge from the Fitzroy River. Modification of the Fitzroy River catchment including the introduction of sheep and cattle from the 1840s (Seabrook et al. 2006; Lewis et al. 2007), extensive clearing of the Brigalow lands from the 1950s (Lloyd 1984; Wilson et al. 2002; Seabrook et al. 2006), mining (predominately coal and gold) and intensive agricultural development (mainly for broadacre cropping) has caused an increase in run‐off as well as loads of suspended sediments and nutrients delivered to the GBR (Kroon et al. 2012; Dougall et al. 2014). However, the effects of these elevated constituent loads within Keppel Bay and beyond are less clear and considerable research on the sources, transport, fate and risk of suspended sediments has been undertaken over the past three decades. While Webster and Ford (2010) synthesised some of this work, their study particularly focussed on the Fitzroy River estuary and did not fully capture the sediment dynamics operating across the catchment to GBR lagoon. In turn, Brooke et al. (2006) produced a comprehensive sediment budget for the Fitzroy