During the most recent glaciation of the Last Glacial Maximum, which ended around 18,000 years ago, the North Sea and much of the British Isles were covered with glacial ice and the sea level was about lower.
One big river system found by 3D seismic survey was the "Shotton River", which drained the south-east part of the Dogger Bank hill area into the east end of the Outer Silver Pit lake.