Gully erosion can be a major disruptor to global fluvial sediment budgets. Gully erosion in the catchments of the Great Barrier Reef is attributed to
Gully erosion is a significant contributor to the increase in global soil erosion rates and is a major driver of suspended sediment-related impacts on downstream aquatic systems (Poesen, 2011; Bartely et al., 2020). This is particularly relevant for water quality conditions in the Great Barrier Reef (GBR), which are negatively impacted by fluvially sourced pollutants, primarily suspended sediment, dissolved and particulate nutrients, and agrochemicals (Waterhouse et al., 2018; Bartley et al., 2014; Brodie et al., 2012; Fabricius et al., 2005). Land use change, such as mining, agriculture (grazing and cropping), and urbanisation associated with European settlement in the region since the 1860s has increased the output of fine sediment and nutrients from the catchments draining into the GBR (Bartley et al., 2018; Kroon et al., 2016). Catchment-tracing studies have consistently identified sub-surface erosion processes, particularly from stream banks and gullies, as being the dominant source of fine sediment delivered to the GBR (Olley et al., 2013; Wilkinson et al., 2015a). Gully erosion in particular has been identified as the largest single source of suspended sediment, estimated to contribute more than 40 % of all fluvially transported sediment entering the GBR (McCloskey et al., 2017). Recent research suggests that these sediments, particularly from grazing lands, also act as a source of bioavailable nitrogen (Garzon-Garcia et al., 2018a, b).
Gullying occurs when unconsolidated soils and sediments become exposed and
eroded by fast-flowing storm runoff (Brooks et al., 2018; Casalí et al., 2009). Gully erosion is a natural process; however, land use changes have increased the rate of gully erosion and the subsequent sediment export (Prosser and Slade, 1994; Shellberg et al., 2016). The tropical climate of the GBR catchment region creates intense rainfall events (often
A recent review by Bartely et al. (2020) identified several scientific studies that evaluated the effectiveness of gully remediation on improving water quality in various regions around the world, including the French Alps (Mathys et al., 2003), southern regions of the United States of America (Polyakov et al., 2014; Nichols et al., 2016), Spain (Hevia et al., 2014), China (Rustomji et al., 2008; Wang et al., 2011), and Ethiopia (Ayele et al., 2018; Dagnew et al., 2015). Bartely and co-workers (2020) concluded that remediation efforts generally decrease the sediment yield of eroding gullies and, thus, improve water quality conditions. However, water quality improvements were driven by the extent of remediation (catchment and gully) and the re-establishment of vegetation in the gully post-remediation (Bartely et al., 2020). Until recently, studies of gully remediation effectiveness in GBR catchments have focussed on smaller-scale gullies (i.e. hillslope gullies), with the application of low-intensity erosion controls such as cattle exclusion fencing, revegetation, and the manual installation of tree branch and/or geotextile fabric check dams (Bartley et al., 2017; Wilkinson et al., 2013, 2015b, 2018). These strategies are effective at reducing erosion in smaller gullies; however, they are not well suited for stabilising the much larger alluvial gullies that are present in many GBR catchments.
Recent research suggests alluvial gullies in GBR catchments require the intervention of intensive landscape-scale remedial efforts to stem further erosion and reduce sediment export (Brooks et al., 2016b; Brooks et al., 2018; Carey et al., 2015; Howely et al., 2018). There are several alluvial gully erosion mitigation projects currently underway in major GBR catchments (e.g. the Normanby and Burdekin catchments), which are trialling various remedial works, including large-scale earthworks (i.e. reshaping of active gully head scarps and sidewalls), rock chutes (including the application of geotextile matting), rock capping and mulching of potentially erodible soils, and the installation of bed control and water velocity reducing measures (e.g. check dams). Stock exclusion and revegetation are also important mitigation measures implemented in these gully remediation projects, often in concert with other treatments. The overall aim of these remedial trials is to ascertain the control measures that are capable of permanently reducing alluvial gully erosion and associated sediment and particulate nutrient export (Brooks et al., 2016b, 2018, 2020b; GA, 2019).
We hypothesise that the application of landscape-scale gully erosion control measures (i.e. gully reshaping, soil compaction, rock armouring of channels and banks, and the installation of check dams) will cause a reduction in the suspended sediment and nutrient concentrations at the study site. Here we aim to assess the effectiveness of landscape-scale remediation in improving the water quality of an alluvial gully situated in the tropics of Queensland, Australia, which flows to the Great Barrier Reef. We apply a recently developed gully water quality monitoring approach that facilitates accurate measurements while meeting the financial and operational requirements of monitoring in remote locations. This work, although done on a limited spatial and temporal scale, provides a critical foundation for developing and evaluating landscape-scale remediation of alluvial gullies in the Great Barrier Reef region.
The study site is located on a cattle station in the Cape York Peninsula region of Queensland, Australia. There are several gullies that have formed
in the alluvial floodplain and terrace of the Laura River (Fig. 1). The tropical climate of the region is characterised by wet (October to April)
and dry (May to September) seasons. Approximately 95 % of the annual rainfall (regional mean annual rainfall is 936 mm) occurs during the wet
season (Brooks et al., 2014a; BOM, 2020). The study site topography is
relatively flat (average slope of the gullies and respective catchments at
the site ranges from 8.6 to 9.7 m m
Topographic map of the study site, including surface geology and gully locations. Source: Geoscience Australia (2019). PASS – pumped active suspended sediment (PASS) sampler; overland PASS – a PASS sampler used to sample water flowing overland (i.e. runoff); rising stage – single-stage sampler (i.e. rising stage sampler). Note: the overland PASS sampler in sub-catchment 3 of the remediated gully was deployed several metres away from the flow line inferred by surface geology due to the redirection of flow associated with vegetation and termite mounds.
A total of two gullies were used to evaluate the effectiveness of the remediation works. The remediated gully is the larger of the two, which encompasses several gully lobes that drain into a central channel. The gully treatment area is
The remediation of the larger gully complex was designed to halt the highly active erosion within the rapidly incising part of the gully and slow the scalding and sheet erosion processes within the broader gully catchment through destocking and the construction of contour berms (Brooks et al., 2018).
The large actively eroding alluvial gully complex was remediated using various intensive, landscape-scale gully erosion control earthworks during
the 2016 dry season. The entire gully complex was regraded and compacted
using heavy machinery. Gypsum was added during this process to reduce soil
dispersibility (Liu and She, 2017), and geofabric covering was applied over
the former gully head scarp and held in place by a coarse sandstone surface
capping. The rest of the gully complex was capped with locally sourced shale
rock. Check dams were installed at regular intervals (approximately every 40 m) in the three major channels that replaced the original gully lobes
(Sect. S1 in the Supplement). After this, the entire gully complex was seeded with native vegetation and livestock were excluded from the gully and its surrounding catchment. No remedial efforts were applied to the control gully, other than the exclusion of livestock (Sect. S1; Brooks et al., 2018). Time-lapse footage of the remedial works is available online at
While gullies commonly share similar patterns of formation and erosion, there are many variables that need to be considered before implementing a monitoring plan to evaluate water quality within a gully system. Ideally, it is best to identify the factors that will have the greatest influence on
gully water quality and monitor them prior to any remediation in order to
establish a baseline of water quality conditions (i.e. a standard before–after–control–impact (BACI) design). Any water quality monitoring assessment of a gully, particularly those being used to evaluate the effectiveness of remediation efforts, should provide a representative measure of the following parameters:
Rainfall – the primary driver of continued gully erosion (Castillo and Gómez, 2016). Soil – characterising basic soil physico-chemical parameters will aid in understanding the transformation of soil into suspended sediment and how that may affect water quality (Brooks et al., 2016b, 2018).
Water quality – it is recommended that at least two different means of water sample collection/measurement are used to ensure a representative measure of SSC and PSD. Entire flow events should also be monitored, if possible (e.g. a time-integrated sample of an event is most representative). If possible, samples should be collected from water flowing into the point of erosion (i.e. above the head scarp) and within the gully after the point of erosion (i.e. downstream of the head scarp; Doriean et al., 2020).
In this instance, the remediation project was required to implement the treatments and monitor the responses within a 3-year timeframe; thus, a
full BACI design was not possible. Instead, a control/impact design was used
in which remediation effectiveness was evaluated against a nearby, comparable
un-remediated control gully (Sect. S2). A total of three repeat airborne LIDAR surveys were collected over a 6-year period, which enabled the calculation of normalised baseline erosion rates for the two sites, demonstrating the comparability of the treatment and control gullies (Brooks et al., 2016b).
A total of two rainfall gauges (Hydrological Services tipping bucket rain gauges – 0.2 mm per tip – with HOBO data loggers) were placed in the catchments of the remediated and control gullies (Fig. 1). The rain gauges were programmed to provide a near-continuous account of rainfall for the sampling period (2017–2018 and 2018–2019 wet seasons). Water level loggers (in situ Rugged TROLL 100® data loggers) were programmed to measure every 2 min and were secured on the surface of a straight section of channel just downstream of each gully head (Fig. 1). A barometric logger (in situ BaroTROLL® data logger) was placed underneath the remediated gully rainfall gauge and set to record atmospheric pressure every 15 min.
The original monitoring plan to evaluate the water quality conditions, focusing on suspended sediment, was limited by funding and available measurement techniques, which resulted in only the outlets of both gullies being monitored for the first wet season (2017–2018). The successful modification of a recently established suspended sediment monitoring method, the pumped active suspended sediment (PASS) sampler, for operation in gullies (Doriean et al., 2020) allowed the monitoring network to expand spatially and, thus, enabled the monitoring of the time-weighted average (TWA) SSC and PSD of sediment entering each gully from their respective catchments during the 2018–2019 wet season.
A total of four different suspended sediment monitoring methods were used to collect water samples in the gullies, namely PASS samplers (Doriean et al., 2019) modified for gully deployments (Doriean et al., 2020), rising stage (RS) samplers (Edwards et al., 1999), autosamplers (Edwards et al., 1999), and turbidity loggers (Rasmussen et al., 2009; Doriean et al., 2020). Several monitoring methods were used in this study to provide multiple lines of evidence to determine the effectiveness of the remediation activities in reducing suspended sediment and nutrient export and provide insight into the performance of the different monitoring methods. Each of the monitoring methods used in the control and remediated gullies were recently described and comprehensively evaluated by Doriean et al. (2020). The turbidity measurements recorded from the two gullies did not provide useful information for comparison of the gullies, and there were few instances where turbidity measurements correlated with physically collected samples. Therefore, turbidity measurement data collected from the gullies are not reported further here (see Doriean et al., 2020). The TWA SSC and PSD of overland flows (i.e. catchment runoff) into the gullies was measured from samples collected using PASS samplers configured to operate in ephemeral waterways (Doriean et al., 2020). The natural slope of the land flowing into the gullies had several depressions or low points that collected water as it flowed over the land; PASS samplers were installed at these locations with the intake and float switch located 0.09 m above the ground (Sect. S2).
Soil samples were collected as part of the design phase of the gully remediation project (Brooks et al., 2016b, 2018). Soil samples (1–2 kg) were collected from the face and walls of the gullies (i.e. the areas undergoing erosion) using a hand trowel and auger at depths ranging from the surface to 1 m. A total of 21 and 9 samples were collected from the remediated and control gullies, respectively, prior to the remediation activities. The soil samples were analysed for particle size distribution, using the soil hydrometer method (American Society for Testing and Materials (ASTM) standard method 152H; Brooks et al., 2016b). Soil particle size distribution data were composited and treated as an average for the purpose of comparing gully soil to suspended sediment. This was done as soil up to 1 m deep can be eroded into suspended sediment during a flow event (e.g. gully wall collapse can impact large sections of the head scarp and expose deeper erodible soils; Garzon-Garcia et al., 2016a).
Water samples collected from the remediated and control gullies were analysed for suspended sediment concentration using gravimetry (ASTM standard method D 3977-97) and particle size distribution, using laser diffraction spectroscopy (Mastersizer 3000; Malvern Panalytical). Samples were screened using a 2 mm sieve prior to analysis to remove any large debris or detritus. TWA SSC of PASS samples was determined using Eq. (1) as follows:
Sediment used for particle size analysis was not chemically treated and was kept in suspension using mechanical dispersion methods (i.e. a baffled container with an impeller stirrer; Doriean et al., 2020). Nutrient analyses were conducted on a select group of samples. The samples were analysed for total and dissolved organic carbon (5310 TOC and DOC 2017), and total and dissolved nitrogen and phosphorus (4500-Norg D and 4500-P B). Dissolved nutrient species (ammonium, oxidised nitrogen, and phosphate) were analysed using the following segmented flow analysis methods: 4500-NH3, 4500-NO3, and 4500-P (APHA 2005; Garzon-Garcia et al., 2015, 2018c). Due to the remoteness of the field sites and the sporadic nature of flow events, it was only possible to retrieve nutrient samples from the autosampler within 48 h of the initial collection on 24 January 2018 and 6 February 2019. Nutrient samples were not retrieved from the other instruments (Manual, RS, or PASS samplers) because the samplers contained samples from previous flow events, or the samples could not be collected and processed within the 48 h time frame. Consequently, the percentage of sand was likely underestimated in the samples, collected by the autosampler, which were analysed for nutrients (Doriean et al., 2020).
GraphPad Prism® was used for statistical analysis of the sample data following an evaluation of the equality of group variances, using the Brown–Forsythe and Bartlett tests before an analysis using paired
Throughout this study, we attempt to acknowledge the uncertainty associated with the various monitoring techniques. A previous evaluation of the sample collection methods was used during this study to determine the approximate uncertainty associated with each method (Table 1; Doriean et al., 2020). These uncertainties were accounted for when interpreting data from the various methods.
Uncertainties, of either SSC or PSD measurements, associated with suspended sediment monitoring methods used in alluvial gullies. Source: Doriean et al. (2020).
Repeat airborne and terrestrial lidar imaging suggest the erosion controls
deployed in the remediated gully had no significant failures, and that sediments are being retained behind the check dams (Fig. 2). Samples were
collected from approximately half (five to six) of all flow events (
Before and after photos (left column) of the remediated gully and repeat lidar images (right column) of the remediated gully for the years 2016 (unremediated), 2017 (post-remediation – 1 year), and 2019 (post-remediation – 4 years). Note the aggradation of sediment in the gully. Green dots show the locations of the photographs. The information in this figure has been modified from Brooks et al. (2020b).
Rainfall totals at the study site for the 2017–2018 (920 mm) and 2018–2019 (915 mm) wet seasons were not significantly different from the yearly average (
Soil characteristics and erosion estimates for the control and remediated gullies (prior to remediation) based on catchment size, area of readily erodible gully soil, and repeat lidar aerial measurements suggest that the control and remediated gullies likely had similar suspended sediment dynamics (Brooks et al., 2013, 2016b). The following sections describe how PSD, SSC, and most nutrient concentrations of samples collected from the remediated gully were significantly different and lower than the control gully for both wet seasons (2017–2018 and 2018–2019). A time series of all monitored flow events is included as supporting information (Sect. S3).
The remote location and challenging monitoring conditions which are typical of alluvial gullies meant that multiple suspended sediment sampling methods were used to ensure that the most representative data were collected throughout both wet seasons (Doriean et al., 2020). Overall, the SSC range of samples collected by each method from the outlet of the remediated gully were significantly lower compared to those collected from the outlet of the actively eroding control gully (Table 2).
Descriptive statistics of SSC samples collected from the control and remediated gullies during the 2017–2018 and 2018–2019 wet seasons.
AS – autosampler; FPS – flow proportional sampling; RSS – rising stage sampler; PASSS – PASS sampler.
PASS sampler data were used to compare time-weighted average (TWA) SSC and other suspended sediment characteristics (i.e. PSD and SSC by sediment particle size class) of the remediated and control gullies because the method collected samples with the most representative PSD and TWA SSCs (Doriean et al., 2020) and monitored the most flow events during both wet seasons (Sect. S3). The low temporal resolution of PASS sample data, theoretically, allows for the potential underestimation of SSC when very high SSCs are present at high flow rates for only short periods over the duration of a flow event (Doriean et al., 2019). However, comparable SSC data collected by manual flow proportional sampling, autosamplers, and RS sampler methods, which have high temporal resolution, corresponded well with the SSC range of the PASS samples from both gullies (Table 2), indicating that the PASS samples were representative of the measured events.
The median TWA SSC of PASS samples collected from the control gully (
Median SSC by sediment size class for PASS samples collected from the control (brown) and remediated (blue) gullies during the 2017–2018 and 2018–2019 wet seasons. Error bars represent the sample standard deviation. Autosampler and RS sampler SSC by PSD are included in Sect. S7.
Time-weighted average suspended sediment concentration and particle
size distribution data of samples collected, using PASS samplers, from the
remediated and control gullies during the 2017–2018 and 2018–2019 wet seasons. Note that the catchment samples (
Please note that each catchment PASS sample TWA SSC represents the average SSC of several flow events.
Relationship between SSC and stream height for single flow events
in the control (
There is currently insufficient water discharge data to accurately estimate
the sediment loads of the two gullies monitored in this study. The unstable
nature of gully banks and bed features means the channel cross-section can
change dramatically during a single event, thus obtaining an accurate measurement of the gully channel cross section over a wet-season is rarely
feasible. As a result, the use of a discharge related rating curve based on
a single measure of channel cross-section will have high uncertainty (Malmon
et al., 2007). Furthermore, manual measurements of water velocity are
dangerous due to the risk of rapid water level rise (e.g. the control and
remediated gullies often encounter water level changes of 0.5 m in under 5 min) and the potential for bank collapse in the control gully. Automated
methods for determining velocity or discharge (e.g. acoustic doppler
velocimeters/acoustic doppler current profilers) offer an alternative to
manual measurements, however, these methods are expensive and are limited to
waters where SSC is typically less than 15 000 mg L
In the absence of water velocity data, comparison of water levels (and thus shear stress), likely to show similar trends to velocity and SSC, show that there was no obvious relationship for the control gully. However, SSC trends in the remediated gully, particularly in the 2018/19 wet season, may be linked to water level, likely as a function of velocity (Fig. 4; Sect. S3). Additional flow event data, including water velocity measurements are needed to confirm this.
The SSC of samples collected from the control gully, using RS samplers and
autosamplers, suggest there is a general decreasing trend in SSC following
the initiation of flow (
Relationship between time after initiation of flow and SSC of samples collected from the control (brown) and remediated (blue) gullies using autosamplers and RS samplers during the 2017–2018 and 2018–2019 wet seasons. Trend lines represent logarithmic regression models.
The PSD of erodible soil collected from both the control and remediated gullies, prior to remediation, were not significantly different (Sect. S8; Fig. 6). For both gullies,
Average PSDs, by size class, of soil collected from the control
(brown;
Suspended sediment samples from the control gully, collected using a PASS sampler, demonstrate the alteration in PSD of the gully soil when it becomes suspended under flow, mixed with sediment from the catchment and selectively transported downstream (Fig. 7). This change in PSD is expected because the sediment particles will distribute in the water column based on their physical and chemical characteristics, such as shape, size, mass, and affinity to flocculate into composite particles (Vercruysse et al., 2017; Walling and Collins, 2016). Hence, lighter and finer particles (clay and silt) were dominant in the suspended sediment samples. The bulk of the sand in the eroded gully soil is likely transported as bed load, with the proportion in the suspended fraction dependant on periods of high flow-velocity (Horowitz, 2008). The presence of large deposits of sand within the control gully channel bed supports this interpretation (Sect. S9).
Control gully soil (brown;
Comparison of the average PSD of suspended sediment samples collected from
the remediated and control gullies show that silt and clay were dominant in
both, however, sand was almost completely absent (
Average suspended sediment PSD by sediment size class
The PSD of control gully catchment PASS samples shifted to smaller sizes
compared to the gully outlet PASS samples (Table 3), which indicates that
the contribution of slightly coarser suspended sediment from gully erosion
(
Average PSDs of PASS samples collected from the remediated gully (blue) and catchments (orange, yellow, and black) and control gully (brown) and catchment (purple) suspended sediment PSD frequency plots during the 2018–2019 wet season. Shading around gully PSDs represents the error as a standard deviation.
Three opportunities occurred during the study period (24 January 2018, 15 December 2018, and 5 February 2019) where samples were able to be retrieved from the remote sampling site within a time frame that allowed them to be processed (i.e. refrigerated, and samples filtered and frozen within 48 h of collection) and analysed. A total of 40 samples were collected from the remediated (
The bulk of total organic carbon and nutrient (nitrogen and phosphorus) concentrations, for both gullies, consisted of particulate fractions (Fig. 10). Organic carbon and nutrient concentrations of samples collected from the remediated gully were significantly lower than control gully samples for both dissolved and particulate fractions, except for dissolved organic carbon and nitrogen during the 2018/19 wet season (Table S10; Fig. 10).
SSC and nutrient concentrations of samples collected during flow
events in the 2017–2018 and 2018–2019 wet seasons. Note that the 2017–2018 data represent a single flow event, and the 2018–2019 data represent multiple flow events. Box plots represent the minimum, maximum, 25th and 75th percentiles, median (horizontal line in box), and mean (cross). Brackets above the box and whisker plots represent the results of paired
Dissolved nutrients are influenced by numerous biogeochemical processes that occur in the catchment and the gully, with some of these processes occurring rapidly (i.e. instantly or within several minutes) and significantly altering nutrient chemical speciation (Garzon-Garcia et al., 2015, 2016b; Lloyd et al., 2019). We do not currently have sufficient information to investigate the effect these processes have on dissolved nutrient trends occurring in the gullies and their catchments, thus, our interpretation of this data will be limited. However, particulate nutrients and carbon are more stable, taking days or weeks to undergo large changes due to biogeochemical processes once initial leaching of soluble components has occurred (Garzon-Garcia et al., 2018a; Waterhouse et al., 2018). Therefore, we can assume that the particulate nutrients are relatively stable and representative of their source when sampled from the gully outlet.
For the samples collected during flow events on 23 January 2018, the SSC and particulate nutrient concentrations showed a significant correlation in the control gully (
Relationships between SSC and particulate organic carbon and nutrient concentrations in the control (brown) and remediated gully (blue) from single flow events on the same day during 2017–2018 wet season.
The large investment in monitoring effort reported in this study was necessary in order to properly assess the effect of landscape-scale remediation on alluvial gully water quality and to test the effectiveness of the different monitoring methods. It is imperative that environmental managers apply robust monitoring plans when conducting gully erosion control measures to ensure that their effectiveness is appropriately evaluated. This study identified the following important factors to consider when
implementing a gully water quality monitoring plan:
The combination of a small number of high-cost monitoring methods (i.e. autosamplers) complemented by low-cost automated methods (i.e. RS and PASS samplers) allows for both redundancy and more representative data collection at key monitoring locations, such as gully outlets. For example, the PASS sampler collected samples from events that occurred after the RS and autosamplers were at capacity, the RS samplers provided important information on in-stream suspended sediment heterogeneity over the rising stage, and the autosampler provided important discrete sample data used to evaluate suspended sediment dynamics (e.g. SSC and water-level hysteresis). The application of low-cost methods (e.g. the PASS sampler) allows for the establishment of a wider spatial monitoring network. In this study, the PASS sampler was deployed at several monitoring locations, in both gully catchments and outlets, which would commonly not be a feasible approach with the other runoff monitoring methods. A complete conceptual model of potential inputs and outputs of a gully should be established before monitoring begins. Failure to do so could lead to inconclusive results and a poor evaluation of gully remediation effectiveness. For example, the lack of catchment data for the 2017–2018 wet season needed to be addressed for the following wet seasons in order to account for all the potential influences acting on the suspended sediment dynamics occurring in the gullies.
The water quality data collected during this study, using multiple monitoring methods, support the application of intensive landscape-scale remediation to significantly reduce suspended sediment concentrations in actively eroding gullies. This is accompanied by the added benefit of significant reductions in nutrient (nitrogen and phosphorus) and carbon concentrations in gully discharge. The findings from this study, regarding the longevity of the erosion mitigation controls used as part of the gully remediation works, are considered to be preliminary, pending the results of monitoring data collected from the site over longer timescales (i.e. semi-decadal to decadal). Development of gully flow velocity or discharge measurement capabilities should be conducted to address the current limitations of discharge measurements in these, often remote, locations. Future studies should also investigate the speciation of particulate and dissolved nutrients in remediated and active alluvial gully systems to better understand the effects of landscape-scale gully remediation on the reduction of bioavailable nutrient export.
The data that support the findings of this study are available from the corresponding author upon reasonable request.
The data that support the findings of this study are available from the corresponding author upon reasonable request.
The supplement related to this article is available online at:
NJCD designed the experiments, carried them out, did the sample and data analysis, investigated the data, and prepared the paper with contributions from all co-authors. WWB provided guidance in the experimental design, method development, and interpretation of results and assisted with the preparation of the paper. JRS provided assistance in the method development, carried out field work, and interpreted the results. AGG and JMB provided sample and data analysis, interpreted the results, and assisted with the preparation of the paper. PRT and DTW provided the interpretation of results and assisted with the preparation of the paper. APB provided supervision, offered guidance in the experimental design, carried out field work, interpreted the results, and assisted with the preparation of the paper.
The authors declare that they have no conflict of interest.
The authors acknowledge the traditional owners of the country in which this study was conducted. We thank the Laura Ranger group for their support throughout the field-monitoring component of this study. We also thank William Higham and Michael Goddard, from Cape York Natural Resource Management, for giving us the opportunity to collaborate with them on-site at Crocodile Station. The authors also acknowledge the Managers of Crocodile Station, Roy and Karlene Shepherd.
Funding for this study was provided by the Australian Government National Environmental Science Programme under Project 3.1.7 – Effectiveness of Alluvial Gully Remediation in Great Barrier Reef Catchments. The Queensland Water Modelling Network also provided funding for the nutrient analysis component of this study.
This paper was edited by Christian Stamm and reviewed by Simon Walker and two anonymous referees.