Paleogeographic evolution and avulsion history of the Holocene Rhine-Meuse delta, The Netherlands

Approximately 200,000 lithological borehole descriptions, 1200 14C dates, 36,000 dated archaeological artifacts, and gradients of palaeochannels were used to reconstruct the Holocene evolution of the fluvial part of the Rhine-Meuse delta. Ages of all Holocene channel belts were stored in a Geographical Information System database that enables generation of palaeogeographic 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 and avulsion history of the Rhine-Meuse delta are governed by complex interactions among several factors.These are: (1) Location and shape of the Late Weichselian palaeovalley. In the Early Holocene, rivers were confined to the Late Weichselian valley.When aggradation shifted upstream, the margins of the valley were crossed by newly formed channel belts. (2) Sealevel rise, which resulted in back-filling of the palaeovalley. (3) River channel pattern. In the central-western part of the delta, a straight anastomosed channel pattern with large-scale crevassing developed as a result of sealevel rise and the associated decrease of stream power. (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 in fault zones), especially from 4500-2800 14C yr BP when the rate of sealevel rise had decreased. After 2800 14C yr BP sealevel rise further decreased, and 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, meander wavelenghts increased, which is interpreted as a result of increased bankfull discharge and/or within channel sedimentation. After 2000 14C yr BP both discharge and sediment load increased as a result of human influence. (6) Coastal configuration. The limited number of tidal inlets and extensive peat formation restricted the number of avulsions in the western part of the delta, and enhanced channel reoccupation. (7) Composition of the substrate and river banks. Meandering river channels tended to adhere to the sandy margins of the Late Weichselian palaeovalley, and high channel sinuosity is found in areas where river banks consisted of sand. Peat formation, which was most extensive in the western part of the back-barrier area especially between 4000 and 3000 14C yr BP, more or less fixed the river pattern at that time, hampering avulsions. (8) Marine ingressions, e.g. the 1421 AD St. Elizabeth’s flood caused large-scale erosion in the southwestern part of the fluvial deltaic plain, resulting in a shift of the main drainage to the SW. (9) Human influence. Since about 1100 AD human influence dominated the palaeogeographic evolution. Rivers were embanked and natural avulsions did no longer occur