The area of the Russell-Mulgrave estuary reporting zone can be defined by the pre-cleared extent of mangrove and saltmarsh remnant vegetation which is characterised by species that require periodic inundation of sea water.
This area extends approximately 1.5 km north and 2 km south of the river mouth along the coastline and extends approximately 5.2 km inland. The area includes the confluence of the Russell River and Mulgrave River and several coastal tributaries that drain into the main river channels.
The assessment area of riparian extent extends further up the river than the mangrove and salt marsh communities to the upper tidal limit and includes vegetation types that are more typical of freshwater environments.
Land use within and adjacent to the Russell-Mulgrave estuary area is dominated by grazing in native vegetation in the downstream reaches with cropping more dominant upstream of the confluence on both the Russell and Mulgrave rivers, with some residential development. The localities of Deeral and Bellenden Ker are just west of the estuary area.
The Russell-Mulgrave estuary grade has remained ‘good’ in the 2018-19 reporting period with the score decreasing slightly from 70 to 68.
The diagram below shows the detailed results for each indicator by year. Click on the timeslider to see data from previous years.
The Russell-Mulgrave estuary water quality remained ‘good’ and the score increased from 66 in 2017-18 to 72.
DIN (dissolved inorganic nitrogen) remained the lowest scoring indicator scoring 27 (‘poor’) with monthly median values frequently not meeting guideline values.
Dissolved oxygen declined from ‘good’ to ‘moderate’, with monthly median values often not meeting guideline values.
Chlorophyll a and turbidity both remained ‘very good’ and FRP (filterable reactive phosphorus) improved from ‘good’ to ‘very good’, with most monthly median values meeting guideline values.
Pesticides improved from ‘moderate’ to ‘good’ and scored 70, equating to over 97 percent of species protected for the risk assessment metric.
Water quality grades are based on aquatic ecosystem guidelines for protection of estuarine waters and are not based on load reduction targets for the marine environment. The results, excluding pesticides, are derived from two monitoring sites in the channel at the confluence of the rivers. Pesticide results were based upon monitoring at the same Great Barrier Reef Catchment Loads Monitoring Program sites on the Mulgrave and Russell rivers as used for the basins.
Habitat and hydrology
The habitat and hydrology index is comprised of flow, which is assessed annually, and three longer-term indicator categories that are updated every four years: mangrove and saltmarsh extent, riparian extent (both updated for 2018-19) and fish barriers (update due for 2019-20).
The habitat and hydrology remained ‘good’ although the score decreased from 75 in 2017-18 to 65 due to the change in the flow indicator score.
The mangrove and saltmarsh extent scored 98.0 (‘very good’) with 0.5% loss from pre-clear to 2017. There was no change in extent between the previous assessment for 2013 and the updated assessment for 2017. These results reflect a low level of development within the estuary area.
Riparian extent is assessed to the upper tidal limit of the river which extends upstream of the estuary area that is characterised by mangroves and saltmarsh. Riparian assessment therefore includes areas that have been cleared for agriculture and other developments. The riparian extent scored 24 (‘poor’) with 47% loss from pre-clear to 2017. There was no change in extent between the previous assessment for 2013 and the updated assessment for 2017
Estuary fish barriers for the Russell-Mulgrave estuary are scored 81 (‘very good’) and are graded 'very good’ for barrier density, ‘good’ for percentage of stream length to the first barrier, with an absence of low passability barriers (graded ‘very good’).
For flows the Russell-Mulgrave estuary declined from ‘very good’ in 2017-18 to ‘moderate’. Flows to the Russell-Mulgrave estuary were altered from modelled predevelopment flows which may have affected ecological values for low flows (low flow spawning fish, critical hydraulic habitat, longitudinal connectivity and water quality) and high flows (fisheries production potential) (Stewart-Koster et al. 2018).
However, the divergence from the pre-development modelled flows could be due to lower accuracy of modelled flow data for large flood events, and for low flows being overestimated by the model during the unusually dry conditions that occurred before the wet season.
The fish index is currently under development and will be available in future report cards.