Hinchinbrook Estuary: Wet Tropics Waterway Health, North Queensland

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 2021-22 and the score decreased from 72 in the previous year to 69.

The diagram below shows the detailed results for each indicator by year. Click on the timeslider to see data from previous years.

WATER QUALITY

Water quality remained ‘good’, and the score decreased from 79 to 73 since 2020-21.

Chlorophyll a, was the lowest scoring indicator and declined from ‘good’ to ‘moderate’ meaning monthly median values sometimes met guideline values.

Dissolved oxygen (low) remained ‘good’ with monthly median values often meeting guideline values.

Dissolved inorganic nitrogen (DIN), filterable reactive phosphorus (FRP) and turbidity were unchanged on ‘very good’, with monthly median values mostly meeting guideline values for these three 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 and shoreline habitat updated in 2021-22), riparian extent (updated in 2021-22) and fish barriers (updated for 2020-21). The habitat and hydrology score declined from 71 in 2020-21 to 65 and the grade remained ‘good’.

Mangrove and saltmarsh scored 83 and was graded ‘very good’. Shoreline mangrove habitat scored 83 with a grade of ‘very good’ whilst mangrove and saltmarsh extent was also graded ‘very good’ and scored 83 with 4 per cent loss from pre-clear to 2019.

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 53 (‘moderate’) with 22 per cent loss from pre-clear to 2019. There was no change to mangrove and saltmarsh extent and riparian extent between the previous assessment for 2017 and the updated assessment for 2019.

Estuary fish barriers for the Hinchinbrook Channel declined from 80 (‘good’) to 60 (‘moderate’). The updated assessment of fish barriers for 2021 added 15 verified fish barries to the 18 fish barriers verified in the 2015-16 assessment. This was due to more comprehensive desktop analyses and field surveys. Due to the increase in identified barriers, the grade for barrier density declined from ‘very good’ to good and the percentage of stream length to the first barrier decline from ‘poor’ to ‘very poor’ primarily due to the identification of a barrier on the Hebert River 29 km upstream of the river mouth which is drowned out during higher flows. There were no low passability barriers identified and this measure retained a grade of ‘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.