The area of the Hinchinbrook Channel 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 includes the length of the channel (approximately 45 km) and extends due south along the coastline from the southern mouth of the channel for approximately 20 km, the area also extends west on the mainland for approximately 10 km and east on Hinchinbrook Island for approximately 6 km.
The area has numerous coastal tributaries that drain into the channel from the mainland and from Hinchinbrook Island. The assessment area of riparian extent extends upstream of the Herbert River further than the mangrove and salt marsh communities to the upper tidal limit and includes vegetation types that are more typical of freshwater environments.
On Hinchinbrook Island the estuary area is all within National Park. The estuary area on the mainland is predominantly conservation use (Girrigun National Park) whilst land uses in the area include forestry and pond aquaculture with some cropping and residential development in the southern reaches on the mainland around the Herbert River.
The towns of Cardwell, located at the north of the channel, and Ingham, located to the south of the channel are both on the mainland. The Hinchinbrook Channel is listed in the Directory of (Nationally) Important Wetlands.
The Hinchinbrook Channel grade remained ‘good’ in 2019-20 with the score increasing from 74 to 78, mainly due to a higher water quality score.
The diagram below shows the detailed results for each indicator by year. Click on the timeslider to see data from previous years.
The Hinchinbrook Channel water quality improved from ‘good’ in 2018-19 to ‘very good’, and the score increased from 77 to 85.
Chlorophyll a remained the lowest scoring indicator but increased from 65 to 77 and the grade of ‘good’ was unchanged meaning monthly median values often met guideline values.
Turbidity and dissolved oxygen improved from ‘good’ to ‘very good’, and dissolved inorganic nitrogen (DIN) and filterable reactive phosphorus (FRP) were unchanged on ‘very good’, meaning most monthly median values were meeting guideline values for all four indicators.
Unlike the other estuary zones, Hinchinbrook Channel has connectivity with coastal waters at either end of the channel and this feature may help enhance water quality.
Whilst there is no targeted monitoring of pesticides in the Hinchinbrook Channel, both the Murray River and Herbert River are monitored for pesticides and drain into the north and the south of the channel, respectively. The basin pesticide monitoring data from these rivers can provide insight into pesticide types and risk of waters entering the channel, noting that dilution of river discharge occurs when mixing with the enclosed coastal waters of the channel.
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 are derived from three monitoring sites in the channel.
Habitat and hydrology
The habitat and hydrology index is comprised of 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 2020-21). The habitat and hydrology score was unchanged at 71 and the grade remained ‘good’.
Mangrove and saltmarsh extent scored 84 (‘very good’) with 4.2% loss from pre-clear to 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.
Riparian extent scored 51 (‘moderate’) with 22% loss from pre-clear to 2017. There was no change to mangrove and saltmarsh extent and riparian extent between the previous assessment for 2013 and the updated assessment for 2017, however the updated Regional Ecosystem mapping (Version 5) resulted in a slight increase (< 1%) of the estimated loss of extent from pre-clear for both habitats.
Estuary fish barriers for the Hinchinbrook Channel scored 80 (‘good’) and were graded 'very good’ for barrier density, ‘poor’ for percentage of stream length to the first barrier, and there was an absence of low passability barriers (graded ‘very good’)
The flow indicator could not be assessed for the Hinchinbrook Channel due to the lack of pre-development modelled flow and flow assessment sites, which are required for the analysis.
Although seagrass habitat occurs in Hinchinbrook Channel there is currently no seagrass monitoring in the estuary and a seagrass grade and score is not available.
The fish index is currently under development and will be available in future report cards.