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29-530: The ecoregion includes, from south to north, the drainages of the Greenough , Murchison , Gascoyne , Ashburton , Fortescue , and De Grey rivers. These rivers drain Western Australia's Pilbara and Gascoyne regions. The climate of the ecoregion is tropical and semi-arid. Most of the rivers are intermittent (or ephemeral). The ecoregion has rugged terrain, underlain by ancient Precambrian blocks. Most of

58-445: A beach , the term shoal can be applied to larger geological units that form off a coastline as part of the process of coastal erosion, such as spits and baymouth bars that form across the front of embayments and rias . A tombolo is a bar that forms an isthmus between an island or offshore rock and a mainland shore. In places of reentrance along a coastline (such as inlets , coves , rias, and bays), sediments carried by

87-401: A harbor entrance or river mouth by the deposition of freshwater sediment or by the action of waves on the sea floor or on up-current beaches. Where beaches are suitably mobile, or the river's suspended or bed loads are large enough, deposition can build up a sandbar that completely blocks a river mouth and dams the river. It can be a seasonally natural process of aquatic ecology , causing

116-433: A longshore current will fall out where the current dissipates, forming a spit. An area of water isolated behind a large bar is called a lagoon. Over time, lagoons may silt up, becoming salt marshes . In some cases, shoals may be precursors to beach expansion and dunes formation, providing a source of windblown sediment to augment such beach or dunes landforms. Since prehistoric times, humans have chosen some shoals as

145-447: A considerable range in size, from a length of a few meters in a small stream to marine depositions stretching for hundreds of kilometers along a coastline, often called barrier islands . They are typically composed of sand , although they could be of any granular matter that the moving water has access to and is capable of shifting around (for example, soil , silt , gravel , cobble , shingle , or even boulders ). The grain size of

174-418: A deep lake, that occurs at any depth, or is used as a verb for the process of proceeding from a greater to a lesser depth of water. Shoals are characteristically long and narrow (linear) ridges. They can develop where a stream , river , or ocean current promotes deposition of sediment and granular material , resulting in localized shallowing (shoaling) of the water. Marine shoals also develop either by

203-427: A sloping bank which is shallower at one end than the other, then the shoaling effect will result in the waves slowing more at the shallow end. Thus, the wave fronts will refract, changing direction like light passing through a prism. Refraction also occurs as waves move towards a beach if the waves come in at an angle to the beach, or if the beach slopes more gradually at one end than the other. Sandbars, also known as

232-430: A trough bars, form where the waves are breaking, because the breaking waves set up a shoreward current with a compensating counter-current along the bottom. Sometimes this occurs seaward of a trough (marine landform). Sand carried by the offshore moving bottom current is deposited where the current reaches the wave break. Other longshore bars may lie further offshore, representing the break point of even larger waves, or

261-439: Is a navigation or grounding hazard, with a depth of water of 6 fathoms (11 meters) or less. It therefore applies to a silt accumulation that shallows the entrance to or course of a river, or creek. A bar can form a dangerous obstacle to shipping, preventing access to the river or harbor in poor weather conditions or at some states of the tide . In addition to longshore bars discussed above that are relatively small features of

290-400: Is also used in a number of ways that can be either similar to, or quite different from, how it is used in geologic, geomorphic, and oceanographic literature. Sometimes, the term refers to either any relatively shallow place in a stream , lake , sea , or other body of water; a rocky area on the seafloor within an area mapped for navigation purposes; or, a growth of vegetation on the bottom of

319-427: Is called shoaling , and the waves are said to shoal. The waves may or may not build to the point where they break , depending on how large they were to begin with, and how steep the slope of the beach is. In particular, waves shoal as they pass over submerged sandbanks or reefs. This can be treacherous for boats and ships. Shoaling can also refract waves, so the waves change direction. For example, if waves pass over

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348-436: Is covered by, sand or other unconsolidated material, and rises from the bed of a body of water close to the surface or above it, which poses a danger to navigation. Shoals are also known as sandbanks , sandbars , or gravelbars . Two or more shoals that are either separated by shared troughs or interconnected by past or present sedimentary and hydrographic processes are referred to as a shoal complex . The term shoal

377-546: Is estimated that the southern dune will reach the mouth of the Greenough around 2045, at which point the Greenough may well become even more prone to flooding, or be forced to change its course. The river is dry most of the time, but has a number of permanent pools such as Bootenal Spring. However, the entire basin is prone to severe flooding which has periodically devastated local farming communities. The worst on record occurred in 1888: over 50 square kilometres (19 sq mi)

406-542: The Chapman River . Later, Rudds Gully cut its own path to the ocean, and the Greenough River discharged into it; and later still, the Greenough cut its own path through the limestone ridge at Walkaway, thus forming its own course. The well-documented strong winds at Greenough have resulted in large mobile dune systems both to the south and north of the Greenough River mouth. Both are gradually moving northwards, and it

435-497: The De Grey River. The fish fauna includes both species widespread through Australia and species endemic to the ecoregion. 20 fish species are native to the ecoregion, of which 5, or 25%, are endemic. Widespread species include the bony bream ( Nematalosa erebi ), Hyrtl's catfish ( Neosilurus hyrtlii ), banded grunter ( Amniataba percoides ), and spangled perch ( Leiopotherapon unicolor ). Endemic freshwater species include

464-419: The Greenough are estuarine ; how far upriver the estuary extends is not certain, but probably about 7 km to near Bootenal Springs. The river mouth is barred from the ocean by a sandbar that breaks only in periods of strong flow. Once open, it usually remains open for several months. The bar has now been reinforced with brushwood on the northern side, to force the bar to open only on the southern side, where

493-614: The Murchison River hardyhead ( Craterocephalus cuneiceps ) and golden gudgeon ( Hypseleotris aurea ), which are found in the rivers from the Gascoyne south, and the Fortescue grunter ( Leiopotherapon aheneus ), which is found in the Fortescue and Ashburton rivers. The blind gudgeon ( Milyeringa veritas ) and blind cave eel ( Ophisternon candidum ) are endemic to brackish-water caves on

522-637: The North West Cape. Greenough River The Greenough River / ˈ ɡ r ɛ n ə f / is a river in the Mid West region of Western Australia . Greenough River has its headwaters near Woojalong Hills on the Yilgarn Plateau . It runs in a south-westerly direction through deep valleys for approximately 340 kilometres (211 mi), passing through the Waterloo Ranges before descending to

551-562: The Western Australian coast, after George Bellas Greenough , President of the Royal Geographical Society in 1837 at the time Grey's expedition was equipped. 28°51′00″S 114°38′00″E  /  28.85000°S 114.63333°E  / -28.85000; 114.63333 Sandbar In oceanography , geomorphology , and geoscience , a shoal is a natural submerged ridge , bank , or bar that consists of, or

580-520: The adjacent dune is more stable. Evidence from wind gaps suggests that, thousands of years ago, the Greenough River turned to the north-west a few kilometres further from the coast than it does at present, at the present-day site of Walkaway . It then ran along the inside of a limestone ridge, following what is now the path of the Geraldton–Dongara railway line, to the vicinity of Rudds Gully . Both watercourses then flowed north to discharge into

609-400: The break point at low tide. In Russian tradition of geomorphology , a peresyp is a sandbar that rises above the water level (like a spit ) and separates a liman or a lagoon from the sea. Unlike tombolo bars, a peresyp seldom forms a contiguous strip and usually has one or several channels that connect the liman and the sea. A harbor or river bar is a sedimentary deposit formed at

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638-441: The bridge near Walkaway, Western Australia . The 1953 flood was the biggest in 30 years. The river water is eutrophic and sediment -laden, and there is evidence that this has affected the ecology of the area, with migratory wading birds no longer visiting the floodplain in numbers as large as previously. The Greenough River was named on 8 April 1839 by the explorer George Grey , while on his second disastrous expedition along

667-676: The coastal plain. A few kilometres from the sea it is obstructed by dunes, and turns to the north-west, running along a dune swale parallel with the coast for about 35 km before finally discharging into the Indian Ocean at Cape Burney , about 9 kilometres (6 mi) south of Geraldton, Western Australia . The ten tributaries that contribute to the Greenough are Woojalong Brook, Woolbarka Brook, Bangemall Creek, Urawa River, Kolanadgy Gully, Nangerwalla Creek, Wooderarrung River, Wandin Creek, Kockatea Creek and Wicherina Brook. The lower reaches of

696-465: The ecoregion is over 200 metres in elevation, and the highest elevation is Mount Meharry at 1,253 metres elevation. Many of the rivers have carved deep canyons as they drain towards the Indian Ocean . During extended dry periods the rivers are reduced to shallow pools in protected canyons. The ecoregion adjoins the Great Sandy Desert to the north and east. It adjoins Southwestern Australia to

725-422: The formation of estuaries and wetlands in the lower course of the river. This situation will persist until the bar is eroded by the sea, or the dammed river develops sufficient head to break through the bar. The formation of harbor bars that prevent access for boats and shipping can be the result of: In a nautical sense, a bar is a shoal, similar to a reef : a shallow formation of (usually) sand that

754-462: The in-place drowning of barrier islands as the result of episodic sea level rise or by the erosion and submergence of inactive delta lobes . Shoals can appear as a coastal landform in the sea , where they are classified as a type of ocean bank , or as fluvial landforms in rivers, streams, and lakes . A shoal–sandbar may seasonally separate a smaller body of water from the sea, such as: The term bar can apply to landform features spanning

783-408: The material comprising a bar is related to the size of the waves or the strength of the currents moving the material, but the availability of material to be worked by waves and currents is also important. Wave shoaling is the process when surface waves move towards shallow water, such as a beach, they slow down, their wave height increases and the distance between waves decreases. This behavior

812-576: The south, which has a more temperate and moist climate, with more year-round rivers. There are extensive cave systems on the North West Cape , with subterranean brackish water habitats hosting unique aquatic communities. The ecoregion corresponds to the Indian Ocean drainage division and mostly corresponds to Pilbara–Gascoyne Level 1 drainage basin as defined in Australia's National Catchment Boundaries (NCB) system, although Pilbara–Gascoyne excludes

841-447: Was flooded; the water was over 5 metres (16 ft) deep south of Greenough; four people were drowned and many financially ruined; and ultimately the town of Greenough was abandoned. Another flood occurred in 1953 with the river being reported as 100 yards (91 m) past the width of the floods in 1883. An 8 feet (2 m) wall of water washed down the river then steadily rose until it was 20 feet (6 m) high, completely covering

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