Gulf of Papua
Coordinates: 9°0′S 145°0′E / 9.000°S 145.000°E
The Gulf of Papua is a 400 kilometer wide region on the south coast of New Guinea.
Some of New Guinea's largest rivers, such as the Fly River, Turama River, Kikori River and Purari River, flow into the gulf, making it a large delta. While the western coast is characterized by swampy tidal waterways, land to the east ending at Cape Possession is flat and sandy.
The Papuan Gulf's central and eastern interior slowly rises to meet the mountainous Southern Highlands, and is covered in a variety of inland swamps and dense tropical hardwood forests. The western interior possess a large region of limestone karst. The dry season begins in October and extends to February, after which the wet season starts.
The southern border of the gulf is defined as a line from the southwestern corner of the Fly River Delta in the west, to Cape Suckling 355 km east of this, which is 70 km northwest of Port Moresby. This encloses a sea area of approximately 35,000 km². The Gulf of Papua forms a broad shelf with a maximum width of about 150 km between the shelf break, at about 140 m water depth, and the Fly Delta. It narrows to less than 20 km east of the Purari River delta. In the southwest the Gulf adjoins the broad, shallow Torres Strait shelf, at the northeastern tip of Australia.
The Gulf of Papua shelf can be divided into four geomorphic zones: a low-relief, inner-shelf, deltaic zone; a high-relief, mid- to outer-shelf incised valley zone; a high-relief, southern reef zone; and a moderate to low-relief mid- to outer shelf zone in the east and northeast. The deltaic zone is an extensive flat, shallow surface in 5–30 m water depth. It is bordered by a relatively steep prodelta region in 20–50 m of water, that extends along the coast between the tide-dominated Fly and wave-dominated Purari River mouths. The incised valley zone has a complex, offshore-onshore-trending seafloor topography on the middle to outer shelf between 50 m and 100 m water depth. Evidence suggests that the largest of these valleys formed by erosive tidal currents during the Pleistocene when sea level was about 30–50 m below its present position. The reef zone is a complex of barrier and patch reefs south of 9° 30' lat., the bathymetry is rugged near steep-sided coral reefs which locally may have vertical sides. Between the reefs, however, the shelf is a mostly flat and featureless surface with water depths of 20–50 m. Water depths increase rapidly to the east of the GBR, which is located on the shelf margin of the Coral Sea basin in water 120–140 m deep. The moderate- to low-relief mid- to outer shelf zone lies north of the incised valleys and offshore from the deltaic zone. Although the 60 m to 80 m isobaths suggest the existence of east-west trending valleys, these are much lower in relief than those to the south. Generally, this zone forms a low-relief plain, gently dipping towards the shelf break which is in about 140 m of water. A drowned barrier reef system forms a rim near the shelf break along the southeastern part of the Gulf.
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