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Dawesville Channel (also known as Dawesville Cut) is a man-made channel between the Peel-Harvey Estuary and the Indian Ocean at Dawesville about 80 km south of Perth in Western Australia. It is just south of the regional city of Mandurah and just north of Yalgorup National Park. The channel was constructed to alleviate an environmental problem which had developed where much of the 136 km², two metre deep estuarine system had become eutrophic. The adjoining estuaries' ability to support the natural flora and fauna had become seriously degraded and the smell of rotting algae, particularly during the summer months had caused increasing complaints from residents. Stocks of the normally abundant fish and crabs had become depleted and recreational use was impacted. The deteriorating environmental situation became a major political and environmental issue for the Government of Western Australia during the mid-1980s. The channel allows seawater from the Indian Ocean to regularly flush in and out of the estuary using the daily tidal movements which in turn prevent the buildup of algae. ==Background== Between May and October of each year, about 1000 mm of rain fall into the catchment areas of the Murray, Serpentine and Harvey Rivers and their tributaries. This represents almost all of the annual rainfall, and so in the drier summer months the rainfall is all but non-existent. This is due to the region's Mediterranean climate. The first European settlers had found the area almost impassable for many months of the year as the flatlands above the estuary become vast floodplains. During the late 1800s, landowners and governments started constructing major drainage systems and culverts to remove the excess water to free up grazing land for livestock and pastures. Forested areas were also cleared. Since then, over 100 years of development of the low-lying sandplain depleted the moisture holding capacity of the soil and any rainfall that fell in the region quickly dissipated into the drains. Soil quality became increasingly degraded and farmers reverted to excessive superphosphate use to compensate for the loss of nutrients. In the early 1970s, several industries, including a large piggery and sheep holding paddocks within the lower catchment poured quantities of nutrients into the river system. A tipping point occurred during the 1970s and 1980s when the discharge of nutrients into the estuary resulted in it becoming eutrophic. Growths of macroalgae in the form of toxic cyanobacteria ''nodularia spumigena'' started occurring on a regular seasonal basis. The combination of sunlight and stagnant, nutrient rich, heated water caused massive blooms of blue-green algae. In the meantime, the region surrounding the estuaries, particularly the coastal strip south of Mandurah was undergoing a major population growth. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Dawesville Channel」の詳細全文を読む スポンサード リンク
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