is joined by Oldcotes Dyke, which flows eastwards from Roche Abbey, and drainage of the surrounding flood plain from here to the Idle is managed by the Rivers Idle and Ryton Internal Drainage Board. A network of drainage channels accompany the river from here to the junction with the River Idle, passing under the A1 road, collecting the runoff from the drainage channels of Whitewater Common, and skirting Serlby and Scrooby. To the north of Scrooby, the channel passes under the A638 Great North Road, the road into Scrooby village and the East Coast Main Line railway, crossing the contour at the railway bridge. The Great North Road bypassed Scrooby, passing to the west of the village, when it was reconstructed as a turnpike road in 1776. The construction included a number of brick arches, to allow the Ryton to flow under it even in times of flood, which can still be seen from Mill Lane. The river used to flow through the village, and powered Scrooby water mill, an 18th-century mill which was used to grind corn. Milling stopped in 1939, and the river was diverted to follow its present course further to the north in the 1960s. Beyond the railway, flood banks have been raised on both sides of the channel until it meets the River Idle just above Bawtry bridge, the limit of navigation on that river. Water quality In order for wildlife, particularly fish, to thrive, the quality of the water needs to be good. The Environment Agency used a six-stage rating scale, from 'A' to 'F', called the General Quality Assessment, to classify rivers. 'A' on the GQA was the best quality of water, while 'F' was 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. Water quality steadily improved as a result of investment in the treatment processes, funded by the water industry's Asset Management Plan. Good flows help the water to purify itself. In 2006, the upper Ryton was rated at 'B' on the GQA scale, and