Changes in the channel-bed level of the eastern Carpathian rivers: Climatic vs. human control over the last 50 years

摘要

This paper is focused on the changes over the past 50 years in the channel beds of seven major rivers that are part of the Siret drainage basin located in eastern Romania. Each river has been subjected to various degrees of human intervention, assessed based on the morphological quality index (MQI): two rivers (Siret and Bistrita) have undergone complex human interventions on 40 and 60%, respectively, of their channel lengths, two rivers (Suceava and Moldova) are 30% artificial, and the remaining three rivers (Trotus, Putna, and Buzau) have a good or moderate MQI. The changes occurring in channel beds and data on water discharge (Q_w) and sediment load (Q_s were assessed simultaneously for the last 50 years. The bedload was indirectly evaluated as ranging between 5 and 15% of the total sediment load. It was determined that incision was prevalent among the processes acting on the channel-bed sections under investigation (representing 62% of the altered area and changing bed level by between —0.25 and —2.70 m), with aggradation accounting for the remaining 38% (causing changes ranging from +0.15 to +1.25 m). The magnitude of the processes (incision or aggradation) for sections with an MQI ＜ 0.3 was four times higher than for sections with a moderate or good MQI (＞0.3). The pattern of change in the channel-bed processes between 1960 and 2010 for all river categories was as follows: a low rate of incision from 1960 to 1979, followed by a higher rate of incision from 1980 to 1989, and finally a tendency of recovery toward the river's initial state, characterised by a decrease in the incision rate or slight aggradation after 1990. The variable that exhibited the strongest response to climate conditions was water discharge (Q_w), whereas the sediment load (Q_s) was highly responsive to both climatic signals and anthropogenic factors. The sediment load has been instrumental in the adjustments of the channel beds by maintaining a balance between the two controlling factors, nature and man.