Tracing landscape evolution using stream profile analysis along the Iron Gates, Danube River, Romania

Abstract

The Iron Gates is a narrow and deep-incised valley section of the Danube River, cutting through the mountain range formed by the Southern Carpathians and North-Balkan Mountains, forming a transverse drainagebetween the Pannonian and Dacian Basins. In the present work longitudinal profiles of 97 tributaries of the left side of the Danube River are analyzed to provide information about the geomorphologic evolution of the area. The stream length-gradient (SL) index anomalies were used to investigate the relationship between between Danube incision, local lithology and regional tectonic processes affecting the area.

By analyzing the SL index anomalies of the Danube tributaries, 8 erosional levels have been identified. The uppermost two levels (L8 and L7) are located at an altitude ranging along the Iron Gates between 410 and 550 m a.s.l., and 330–490 m a.s.l., respectively. These could be correlated with pre-Danubian planation surfaces, described by previous authors. They show an upwarped elevation pattern along the river, indicative of differential uplift rates with the higher values at the central range of the study area (Almăj Mountains).

For most tributaries, the largest SL index anomaly values were recorded for the subsequent level (L6), which developed at 230–360 m a.s.l. This zone separates the upstream relict profiles from the steeper, rejuvenated downstream valley segments. It corresponds to the “Ciucaru/Kazan” level, the lowest well developed planation surface above the incised gorge, equivalent of the highest strath terrace of the Danube. Its abandonment by the incising Danube River means the onset of the antecedent formation of the Iron Gates. The knickzones below the „Ciucaru/Kazan” level along tributaries reflect successive phases of incision of the streams, as adaptations to the local base level drop following the incising Danube. The upwarped pattern of the knickzone levels along the gorge (being the highest in the Almăj Mountains) is gradually faded towards the lower levels. The lithology only slightly influenced the occurrence of slope anomalies and the concavity of longitudinal profiles, but without an essential contribution regarding the spatial pattern of erosional levels.