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 280 square miles (730 km2), which has an average annual rainfall of 24 inches (610 mm) (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.
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