Holocene floodplain evolution of small rivers in the uplands of Lower Saxony, Germany

Abstract

The Holocene floodplain evolution of the river Ilme and its distal tributaries in the uplands of Lower Saxony (Germany) has been reconstructed by shallow corings arranged in cross and longitudinal profiles. In those river reaches that drain agricultural land we differentiate three Holocene fluvial units overlying the Weichselian Lower Terrace: A fine-grained Humus Basal Layer (early-middle Holocene), an Older Meadow Loam (8th-early 14th century) and a Younger Meadow Loam (since 15th century). Intertongued with these sediments are alluvial fans and colluvia of two periods: 1. Neolithic to early Medieval time, 2. 14th century. In contrast forested floodplains are underlain by sand and gravel, whereas on floodplains used as meadows, a fine-grained “Wiesensediment” (Meadow Sediment) has accumulated since the 17th century. The Humus Basal Layer and the sandy gravel were both accumulated by braided rivers. In the middle and lower reaches of the Ilme floodplain, the change to a meandering river happened when the Older Meadow Loam accumulated as a result of woodland clearing and soil erosion in the Early Middle Ages. In the course of time, the vertical accretion of the floodplain decreased as a result of the decreasing frequency of inundations, itself caused by the increasing capacity of the river channel. Vertical accretion was nearly terminated in the Late Middle Ages when a short climatic change caused fan and colluvial deposition. Since the 15th century, the Younger Meadow Loam accumulated, most of it filling old channels. In the wooded Solling hills, the change-over from a braided river to a meandering river began in the 17th century, but in those reaches with wooded floodplains the rivers are still braided.