Late Holocene history of the Moldova River Valley, Romania

This paper describes the results of geomorphological, sedimentological and 14C analyses of Late Holocene fluvial sequences in the Moldova River Valley of Romania. This river originates from the Carpathians mountains and is known for the extent of its large floodplain and sediment thickness. Between the Molid and Timişeşti localities (river length: 86 km), numerous large fossil trunks of oak, poplar and beech trees are exposed in the riverbanks. Six wood samples were selected for radiocarbon absolute dating (conventional method), and their ages ranged from 410 to 3200 cal yrs. BP. The thickness of the alluvial sediments above the sampled fossil trunks varied between 2 and 3 m, and these sediments predominantly consisted of coarse gravel materials that had similar granulometry to sediments that were present in the modern riverbed. Additionally, in the same investigated areas, successive floodplain cross-sections were constructed based on more than 20 hydrogeological boreholes. For each cross section, all data were used to calculate the local values of bed loads, and these values were compared to the absolute ages of the fossil trunks and were in the range of 300 m3/m and 5500 m3/m. Based on the calculated values, during the last 780 years, the floodplain sedimentation rates were estimated to be in the range of 0.4 m3/year at the Molid Section, which was located in the mountainous area of the valley and 44 km downstream from the river headwater, and during the last 410 years, the sedimentation rate at the Praxia Section, which was located 125 km downstream from the river headwater, was 13.3 m3/year. In the past 100 years, the fluvial processes along the Moldova River were dominated by a narrowing (by an average of 76%) and an incision (up to 2.5 m), and a channel metamorphosis from braided to wandering was reported. These results are discussed in the context of Late Holocene regional and local palaeoclimatic reconstructions as well as that of human interventions.