Late Weichselian and Holocene palaeogeography of the Rhine–Meuse delta, The Netherlands
Abstract A large number of lithological borehole descriptions, 14C dates, archaeological artefacts, and gradients of palaeochannels were used to reconstruct the Holocene development of the Rhine–Meuse delta. Data were stored in a Geographical Information System database that enables generation of palaeogeographical maps for any time during the Holocene. The time resolution of the palaeogeographic reconstruction is about 200 years. During the Holocene, avulsion was an important process, resulting in frequent shifts of areas of clastic sedimentation. Palaeogeographic evolution of the Rhine–Meuse delta is mainly governed by complex interactions among several factors. These are: (1) Location and shape of the Late Weichselian palaeovalley. (2) Sea-level rise, which resulted in back-filling of the palaeovalley. (3) Peat formation, which was most extensive in the western part of the back-barrier area especially between 4000 and 3000 14C yr BP. This more or less fixed the river pattern at that time and resulted in few avulsions. (4) Neotectonics. Differential tectonic movements of the Peel Horst and Roer Valley Graben seem to have influenced river behaviour (formation of an asymmetrical meander belt, location of avulsion nodes), especially from 4500 to 2800 14C yr BP when the rate of sea-level rise had decreased. After 2800 14C yr BP, the sea-level rise further decreased, and the tectonic influence still may have influenced avulsions, but from then on, other factors became dominant. (5) Increased discharge, sediment load and/or within-channel sedimentation. After 2800 14C yr BP, river meanders of Rhine distributaries as well as the single channel of the Meuse show remarkable increases in wavelength, interpreted as a result of increased bankfull discharge and sediment load. Increased discharge may initially have been caused by a higher precipitation. After 2000 14C yr BP, both discharge and sediment load seem to have increased as a result of human influence. Alternatively, decreasing gradients (as a result of sea-level rise) may have caused an increased within-channel sedimentation and channel widening, which would also lead to increased meander wavelengths. (6) Composition of the river banks. Meandering river channels tend to adhere to the sandy margins of the Late Weichselian palaeovalley, and a high channel sinuosity is found in areas where river banks consist of sand. (7) Marine ingressions, e.g., the AD 1421 St. Elizabeth's flood, caused large-scale erosion in the southwestern part of the fluvial deltaic plain. (8) Human influence. Since about AD 1100 human influence dominated the palaeogeographic evolution. Rivers were embanked and dredged, meanders were cut off, and for the Meuse, a new distributary canal (the Bergsche Maas) was dug in AD 1904. Discharge distribution over the various channels is strictly controlled nowadays. An unusual feature of the delta is the prolonged existence of the Oude Rijn channel belt.