that drops the level again, before the river passes under West Bridge on the A614 Worksop to Ollerton road. Most of the rest of the route is through woodland, in the middle of which a ford crosses the river near Crookford farm. The river is crossed by the B6387 Ollerton to Gamston road, and the A1 road near Gamston Airport, before joining the River Idle just downstream of the point where the River Maun and River Meden meet and the Idle begins. Hydrology The source of the Poulter is in a region where the underlying geology is an extensive water-bearing porous rock structure called the Magnesian Limestone aquifer. Magnesian limestone is named because if contains quantities of the mineral Dolomite, which is rich in Magnesium. Where this rock reaches the surface, springs often occur, and the Poulter starts at such a spring. Further to the east, the Magnesian Limestone is covered by a layer of porous rock called the Triassic Sherwood Sandstone aquifer, which is the major geological component of the area. Continuing eastwards, both are covered by a layer of Mercia mudstone. The groundwater levels in these aquifers affect whether they supply water to the river system, or take water from it, and levels are lower than they have historically been, as a result of 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. Above Cuckney, where the Environment Agency have a gauging station which records flows in the river, the catchment for the River Poulter 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 river. Water quality is good. The Environment Agency use a six-stage rating scale, from 'A' to 'F', 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