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The River Idle is a river in Nottinghamshire, England. Its source is the confluence of the River Maun and River Meden, near Markham Moor. From there, it flows north through Retford and Bawtry before entering the River Trent at Stockwith near Misterton. The county boundary with South Yorkshire follows the river for a short distance near Bawtry, and the border with Lincolnshire does the same at Idle Stop. Originally, it flowed northwards from Idle Stop to meet the River Don on Hatfield Chase, but was diverted eastwards by drainage engineers in 1628. Most of the land surrounding the river is a broad flood plain. Between Retford and Bawtry, the floodplain is partly occupied by a number of sand and gravel pits, where exhausted forming public lakes for fishing, while beyond Bawtry, the river is constrained by high flood banks, to allow the low lying areas to be drained for agriculture. Its main tributaries are the River Poulter and the River Ryton. The river is navigable to Bawtry, although navigation rights were removed in 1972. It is also important for conservation, with the Idle Washlands and some of the sand and gravel pits of the Idle Valley being designated Sites of Special Scientific Interest. ==Hydrology== To the west of the region through which the river flows, the underlying geology is an extensive water-bearing porous rock structure called the Magnesian Limestone aquifer. Magnesian limestone is so named because if contains quantities of the mineral Dolomite, which is rich in Magnesium. Further to the east, this rock is covered by another layer of porous rock called the Triassic Sherwood Sandstone aquifer, which is the major geological component of the area. Continuing eastwards, both are then covered by a layer of Mercia mudstone. Where these aquifers reach the surface, they often supply water to the river system, but can also take water from it. This is affected by the extraction of groundwater, particularly for public water supply, and by fracturing of the aquifers as a result of subsidence caused by deep coal mining. The catchment for the River Idle covers some , which has an average annual rainfall of (based on figures from 1961 to 1990). About a third of this finds its way into the rivers.〔 Water quality is moderate. The Environment Agency use a six-stage rating scale, from 'A' to 'F', called the General Quality Assessment, to classify rivers. 'A' on the GQA is the best quality of water, while 'F' is the poorest. Factors which affect the quality are levels of ammonia, levels of dissolved oxygen and the Biochemical oxygen demand (BOD), which measures the amount of dissolved oxygen needed by organisms to break down organic matter in the water. These factors are generally worse when the water is discharged from sewage treatment processes, and tend to be worse in summer, when such discharges make up a greater proportion of the total flow in the river. The tributaries of the Idle are rated at 'C' on the GQA scale, because they pass through urban areas, and there are significant discharges to the rivers from sewage treatment works. The average flow in the River Maun is around 13 Mld (megalitres per day) in dry weather, which is supplemented by Mansfield sewage treatment works, which discharges nearly 23 Mld. Water quality does not improve in the Idle, and remains at 'C' on the GQA scale all the way to the Trent. The Environment Agency maintain gauging stations to measure the flow in the river near the junction with the River Poulter, and to the east of Mattersey. 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「River Idle」の詳細全文を読む スポンサード リンク
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