Source area lithological control on sediment delivery ratio in Trotuş drainage basin (Eastern Carpathians)

Abstract

Depending on their response to erosion, we documented the occurrence of three distinct lithological groups within Trotur drainage basin. The classification criteria were: the petrographic composition of geological formations, rock hardness, the relative relief and the longitudinal profile gradients for 2^nd and 3^rd order streams in the Strahler classification system. The two parameters employed in computing the sediment delivery ratio (i.e. the gross erosion and the sediment yield) were estimated for each lithological group based on the multiple regression equations introduced by Ichim & alii (1986). The aggregation of gross erosion estimates for each lithological group resulted in a total amount of approx. 107×10⁵ t of sediments eroded annually from the entire area of Trotur drainage basin, of which 0.7×10⁵ tyr^-1 were removed from the erosion resistant rocks area, 20.3×10⁵ tyr^-1 from the area with moderate resistance to erosion, and 86×10⁵ tyr^-1 from the area with low resistance to erosion. The sediment yield derived by using multiple regression was converted to the specific sediment yield, thus obtaining the following classes: 39-50 tkm^–2yr^–1 for high resistance areas; 220-350 tkm^-2yr^-1 for moderate resistance areas, and 800-1,900 tkm^-2yr^-1 for areas with low resistance to erosion. The values of the sediment delivery ratio reveal the increasingly larger sediment storage as we approach the junction with Siret river. Thus, in Lunca de Sus gauging station (located 14 km from the headwater) the delivery ratio is as high as 40%, as compared to 23% in Goioasa (54 km from the headwater), 15% in Târgu Ocna (90 km from the headwater), and just 7.6% of the total amount of eroded sediment evacuated annually in the drainage basin outlet.