Ecological systems

Mangroves and salt marshes

Mangroves are salt-tolerant trees and shrubs that are adapted to live in harsh coastal conditions. They contain complex salt filtration systems and root systems to cope with saltwater immersion and wave action, and have adapted to the low-oxygen conditions of waterlogged mud.

The Hawkesbury supports two species of mangroves; Grey Mangroves (Avicennia marina) and River Mangroves (Aegiceras corniculatum). Grey Mangroves are very tolerant to high levels of salt where as River Mangroves tend to be found further upstream in the fresher reaches. However, the two species can be seen growing side by side, usually with Grey Mangroves along the stream front and River Mangroves behind where they are less susceptible to tidal inundation. 

Salt marshes are coastal wetlands that are flooded and drained by salt water brought in by the tides. They are characterised by small halophytic plants (plants that have adapted to grow in saline conditions) such as grasses, reeds and shrubs. 

Saltmarshes are found in the lower Hawkesbury. Some of the species of plants found in these saltmarshes include Twig-rush (Maumea juncea), Sea Rush (Juncus kraussii) and Austral Seablite (Suada australis). 

Mangroves are important to the Hawkesbury River because they provide shelter for many fish species, including commercially and recreationally important species such as Yellowfin Bream, Sand Whiting and Mullet. Salt marshes found along the River are a critical habitat for many species of wading and waterbirds, many of which are listed as vulnerable or endangered.  Mangroves and salt marshes produce large amounts of organic matter which enter our waterways and are eaten by some small animals. These in turn provide food for larger fish and other animals. Both environments protect the coastal areas of the catchment from erosion by absorbing the energy of tidal currents. They also help maintain water quality as they trap sediments, nutrients and contaminants contained in runoff, which prevents them from entering the River. 

Swamps and wetlands

The swamps of the Hawkesbury River are dominated by paperbarks (Melaleuca) and she-oaks (Casuarina). The understory of paperbark dominated wetlands includes a range of sedges, ferns and shrubs. These wetlands more commonly occur on sandier, alluvial soils and slightly more brackish conditions than the wetland areas dominated by she-oaks. She-oaks tend to be found in more freshwater settings and tend to dominate the riverbanks further upstream along the main stem of the Hawkesbury. 

There are extensive floodplains found along the Hawkesbury River which have formed a significant number of freshwater wetlands of varying sizes. Shallow wetlands that occur in freshwater areas of the Hawkesbury contain a variety of plants that live along the water's edge including Tall Spike Rush (Eleocharis sphacelata), Water Ribbons (Triglochin procerum) and Cumbungi (Typha orientalis). 

Swamps and wetlands are important to the River system because they act as natural flood control, absorbing excess rainwater and preventing floods in adjacent areas. They provide essential habitat for a variety of plant and animal species, fostering biodiversity and supporting various life cycles. Moreover, the plants in wetlands systems act as water purifiers, filtering various pollutants and nutrients.

Dry Sclerophyll Forest 

Like the Wet Sclerophyll Forest, the canopy layer is dominated by Eucalypts. Unlike the Wet Sclerophyll Forest, which contains plants with soft leaves, the understory of Dry Sclerophyll Forests are sparse and dominated by hard, leathery leaved plants that have adapted to prevent moisture loss. These forests tend to grow on poorly fertile sandy soils in areas that recieve between 500 - 900 mm rain annually. There are four types of Sclerophyl Forests that occur in the Hawkesbury catchment. These are listed below with some of their dominant tree species.  

Cumberland Dry Sclerophyll Forest: Grey Box (Eucalyptus moluccana), Forest Red Gum (Eucalyptus tereticornis) and Narrow-leaved Ironbark (Eucalyptus crebra). 

Sydney Coastal Dry Sclerophyll Forest: Sydney Red Gum (Angophora costata), Red Bloodwood (Eucalyptus gummifera) and Old Man Banksia (Banksia serrata).

Sydney Hinterland Dry Sclerophyll Forest: Yellow Bloodwood (Corymbia eximia), Narrow-leafed Apple (Angophora bakeri) and Red Bloodwood (Eucalyptus gummifera). 

Sydney Montane Dry Sclerophyll Forest: Blue Mountain Ash (Eucalyptus oreades) , Sydney Peppermint (Eucalyptus piperita) or Narrow-leaved Peppermint(Eucalyptus radiata). 

Heathlands

Heathlands tend to occur in in nutrient poor soils so as a result, vegetation is stunted typically growing between 30cm to 2m tall. Some of the plants found in these environments include Mallee Eucalypts, small stunted shrubs and Sclerophyllous sedges. 

Terrestrial vegetation plays an important role in the Hawkesbury-Nepean catchment for a number of reasons. The roots, canopy cover and leaf litter provided by trees contribute to better water infiltration. This reduces soil erosion and runoff, which can carry pollutants into the River and its tributaries. Plant root systems also help filter and purify water by trapping sediments and absorbing excess nutrients. Furthermore, terrestrial vegetation provides food and a habitat for the many terrestrial species found within the catchment.

Riparian zones

A riparian zone is the transitional area of land adjacent to rivers, streams, or other water bodies. It's characterized by unique vegetation like water plants, shrubs and trees. Riparian zones are important because they act as natural buffers, preventing soil erosion and filter pollutants from runoff, which helps maintain water quality. They also provide vital habitat for various species, supporting biodiversity, and offering food and shelter to both aquatic and terrestrial wildlife. These areas help regulate water temperature and influence local climates.

A great deal of the riparian zones along the Hawkesbury-Nepean River and its tributaries have been degraded. This is due to the clearing of riparian areas for agriculture, private homes and caravan and ski parks, water development (e.g. jetties and stairs), weed invasion, boat activities and pollutants such as fertilizer. Local councils and organisations such as Streamwatch and Greater Sydney Landcare, are working to upskill local landowners and citizens to better care for riparian zones through education and hands on workshops. Councils are also taking measures to increase surveillance and closely monitor riparian areas for illegal clearing, pollution and dumping. However, there is a great deal more work that needs to be done to improve these areas. 

Weeds

Invasive weeds are a problem within the catchment, particularly in riparian areas. Weeds have replaced native species of flora but provide few of the same benefits. For example, providing habitats and food resources for native fauna, and stabilising river banks. Some examples of invasive weeds found within the catchment include lantana, salvinia, privet, camphor laurel, bitou bush and tobacco weed. 

The Hawkesbury River estuary, plays a crucial role in supporting life due to its high nutrient inputs. These nutrients are essential for the growth of plants and other organisms in the ecosystem. The river and its estuary receives nutrients from various sources, including freshwater flows from within the catchment, exchange with floodplains and tidal habitats, trophic relay through organisms like fish, ocean inputs, and internal recycling.

However, human activities, such as urban and agricultural practices, can significantly increase nutrient inputs, leading to a process called eutrophication. Eutrophication occurs when there is an excessive enrichment of nutrients in the water, causing an overgrowth of undesirable plants like phytoplankton. This overgrowth, which is commonly referred to as algal blooms, can have detrimental effects on the river including;

• High densities of algae can reduce light penetration through the water which prevents the process of photosynthesis, impacting the growth of water plants

• When algae dies, the breakdown of the organic matter consumes oxygen from the water which can lead to anoxia (oxygen deprivation). This can cause the death of aquatic organisms and a decline in biodiversity

• Some species of algae produce toxins which can be harmful to humans and animals.

The Hawkesbury River has experienced increased nutrient inputs since the 1970s, mainly due to its proximity to urban areas and associated sewage treatment plants, as well as urban and agricultural practices on the floodplain. This has led to the degradation of the river's condition in terms of eutrophication. Long-term monitoring over the past 30 years has revealed cleaner sections in the upper and lower parts of the river. However, the middle course of the river, approximately 100 km between Yarramundi and Wisemans Ferry, faces significant nutrient and pollutant challenges.