Cooks River runs into the northern side of Botany Bay. It is shown on Cook's map, evidence that he explored it for some distance, but it was not until the First Fleet's brief stay in 1788 that Governor Phillip and some of the officers explored the River at the NW side of the bay "which we went up for about 6 miles, finding the Country low & boggy, & no appearance of fresh water...". An "arm of the sea" was the succinct description used by Lieutenant William Bradley of the Sirius when he charted the course of the river in December 1789.
The once-extensive estuary of the Cooks River, with its meandering channel and mudflats and mangroves, has now been totally obliterated by the runways of Sydney Airport. The river now reaches Botany Bay along a concrete channel that diverts its course through the sandy shoreline to join Botany Bay west of its original mouth.
It is hard to visualise the original river but perhaps the most tangible evidence of the original vegetation patterns are some plans prepared in the 1880s for the Surveyor-General. These show the once-extensive areas of mud flat dry at low tide, mangrove scrub, salt water swamp (probably saltmarsh) and low scrubby ground.
However, estuarine plants are hardy and vegetation still survives close to the river, on low-lying land subject to periodic flooding or with saline soils that few weeds can survive in. Grey Mangrove, Avicennia marina is the most conspicuous estuarine plant. Its seeds are shed already sprouting and they colonise silt deposits and semi-natural banks. Grey Mangrove shrubs are conspicuous along the river between Tempe and Croydon Park and along Wolli Creek and Muddy Creek. They provide important habitat and shelter for waterbirds, fish and other aquatic animals.
Saltmarsh plants are small and low-growing and have not survived as well as the Grey Mangrove. However, at Barton Park near the soccer stadium, a surprising area of saltmarsh has survived. Dominated by Samphire, Sarcocornia quinqueflora, this is the largest substantial remaining expanse of saltmarsh left on the Cooks River. Saltmarsh plants may take on distinctive coloration, and the Sarcocornia saltmarsh may have attractive purplish tints in winter. Other saltmarsh plants, such as the grass Sporobolus virginicus, Seablite, Suaeda australis and Triglochin striatum may also occur in these sites but are generally rare.
Estuarine wetlands followed Wolli Creek as far as the weir at Turrella, where patches of mangrove and saltmarsh can still be seen. There are localised patches of Sarcocornia quinqueflora plants on the banks of the Cooks River at Gough Whitlam Park at Undercliffe where there is still a natural earth bank. At Marrickville, saltmarsh plants are attempting to colonise the edge of the river.
The large shells of mud oysters and other shells now found along the banks remind us of the plentiful foods that the river once provided for Aboriginal people. These shells probably came from dredged material used as fill when the artificial banks were constructed. Some Aboriginal shell middens may still exist.
See Missing Jigsaw Pieces: the bushplants of the Cooks River Valley by Doug Benson, Danie Ondinea and Virginia Bear (1999), published by the Royal Botanic Garden Sydney, for more information on the history and plant life of the Cooks River.