Map showing extent of Doggerland, a vast territory in northwest Europe that was above sea level 16,000 years ago, and the territory's diminishing size as ice melted and the North Sea rose. Illustration by Claus Lunau/Science Photo Library.
Via https://sciencemastodon.com/@ebender00/113606996477811407
From what I read, those river valleys were very productive hunting areas, filled with rich ecosystems