Tunnel valleys have long fascinated the geoscientists by their scale and the consequent intensity of the sedimentary processes necessary to their formation. The filling of tunnel valleys in the southern North Sea is mainly made of clinoforms prograding north. A new sedimentological model has been developed, stating that the incision and the filling of the valleys are separate in times and from distinct processes. The erosion surface and consequently the tunnel valleys peculiar incision geometry are preserved after ice recession, forming sediment traps. The infill is interpreted as proglacial for the newly observed south-dipping clinoforms and postglacial for the north-dipping clinoforms onlapping the later. The north-dipping clinoforms are interpreted to be formed within a large deltaic system associated with the Rhine-Meuse river(s). The delta was probably infilling a lake containing 100's of m overdeeps (the underfilled tunnel valleys). The presence of clinoforms 50-80 m above the valley shoulders indicates the potential depth of the lake. However, remains of the regionally extensive lake are elusive and seldom preserved onshore. We have hypothesised that the depression hosting the lake was controlled by the lithospheric flexure which is present only during the glacial maximums and consequently difficult to pinpoint by only analysing today’s topography. In this depositional system, a competition exists between one of the biggest river of Europe facing ice sheets and the proglacial supply, generating a very intricate stratigraphy. We intend to solve part of the problem by numerically reconstructing the local landscape with the ice sheet and its isostatic depression. This allows to locate potential depocenters and lake thresholds/spillways to better understand the river migration and the position of the lacustrine systems. The Elsterian reconstructed landscape is unexpected since we show that the orientation of the slopes are locally reversed and the altitudes were drastically different between glacial maximums and interglacial times.