The lake surface rose until it overflowed southwards and cut an exit between the Howardian Hills and the Yorkshire Wolds at Kirkham Priory between Malton and Stamford Bridge, so creating the River Derwent.
In modern times, as an artificial flood relief channel, much of the flow of the River Derwent (which drains a large area of the North York Moors) has been diverted, about 6 miles (10 km) upstream of West Ayton, before it reaches the plain of the Vale of Pickering, east into a new channel called the Sea Cut along a previously dry side valley (probably a glacial overflow channel) and into the existing Scalby Beck's course through Scalby, North Yorkshire to the North Sea.
The idea of these lakes was first proposed in 1902,when Professor Percy Kendall of Leeds University published a paper detailing his theories.
[5][6] With the old exit blocked by the North Sea ice sheet,[7] the Vale of Pickering filled and overflowed between the Howardian Hills and the Yorkshire Wolds into an arm of a much larger proglacial Lake Humber which filled the lower Ouse valley, the lower Trent valley and, via a narrow gap at Lincoln, the Fenland basin.
This is the altitude of the highest point on its spillway, at the head of the River Wissey, a level verifiable by looking for old shore-lines around The Fens.