Late Holocene fluvial activity and correlations with dendrochronology of subfossil trunks: Case studies of northeastern Romania

The main objective of this paper is to describe the late Holocene behaviour of rivers using an interdisciplinary approach combining fluvial geomorphology and subfossil trunk dendrochronology. The subfossil wood material collected along the rivers was investigated for dendrometric and dendrochronologic parameters. The research methods in these fields helped us to understand the effect of the fluvial environment on riparian trees and their records and helped in reconstructing the riparian palaeoenvironment.The study area consists of two rivers with different typologies but comparable sizes: the Moldova River, which features a braided to wandering channel in its lower reach, and the Siret River, which features a sinuous-meandering channel. Along the 100-km-long floodplain of the former and the 144-km-long floodplain of the latter, we found and sampled 77 subfossil trunks, of which 26 were subjected to 14C dating. The resulting data consist of floodplain facies mapping data, electric resistivity measurements, absolute dates, and dendrometric and dendrochronologic data. The results indicate that during a 100-year period, the two rivers were sensitive to climate change and anthropogenic effects, particularly a narrowing of the active channel by 76% in the braided channel and 38% in the sinuous-meandering channel. During the past 3300–3000 YBP, the Moldova River maintained its braided style, whereas the sinuous-meandering style has been characteristic of the Siret River for the previous 6800–4600 YBP. The two distinct fluvial environments are recorded in the dendrometric structure of the trunks buried in the channel-fill sediments. The braided fluvial environment was more effective in uprooting riparian trees and incorporating them in the floodplain deposits, whereas the sinuous-meandering style of stream effectively buried tree trunks in lateral accretion lobes. Absolute and dendrochronologic dating allowed for the reconstruction of timelines of the felling of the trees and estimates of the magnitude of the responsible hydrological event. The flood events on the Siret River with a recurrence interval of 200 years (Qmax ~ 2500–2800 m3/s) were those most effective in destroying riparian forests, and the events on the Moldova floods with a 100-year recurrence interval (Qmax ~ 1200–1400 m3/s) were the most effective. Dendrochronology allowed for identification of wet phases (i.e., 3500–2900 YBP, 2200–2075 YBP, and 1000–800 YBP) and dry phases (e.g., 3200–3150 or 2775–2700 YBP, 1400 YBP). Finally, we draw attention to the potential for creating a highly replicable dendrochronological series spanning at least 7000 YBP.