The main regularities of the ratio between riverbed and basin components of erosion and suspended sediment yield in river basins of the USA

Abstract

Suspended sediment yield is one of the objective and sufficiently accurate measures of erosion intensity in river basins. In first approximation, it can be divided into the riverbed component –r(rb), the products of vertical and horizontal riverbed deformations), and basin component – r(bas), the products of soil and gully erosion. An attempt was made to distinguish this erosion structure in the USA river basins based on the partition of suspended sediments of 224 rivers (based on the data from the US Geological Service on the average monthly water discharges and suspended sediment yields) according to the method proposed by one of the authors of the paper, as well as an assessment of its factor dependence. The average r(rb) value for the analyzed rivers of the USA is 7.9±1.1%: for lowland rivers – 10.6±1.7%, for low-mountain (including uplands) rivers – 5.7±1.5%, for mid-mountain rivers – 4.3±1.5%. The geomorphic factor, landscape and climatic conditions within the river basins have a major impact on the suspended sediments flux ratio r(rb)/r(bas). Thus, in the USA plains, the largest average r(rb) portion is in the forest landscapes (taiga, mixed and broadleaf forests of the temperate zone, subtropical forests) – 10–15%. On the contrary, in the arid landscapes (semi-deserts) this value does not exceed 1%. Within these general trends, there are quite strong variations in the r(rb)/r(bas) ratios due to the changes in high river basin areas, agricultural activities and lithologic composition of the riverbed and floodplain sediments. There is an inverse hyperbolic relationship between the actual suspended sediment yield of rivers and the riverbed sediment portion (r(rb)), which is most manifested in the plains and low-mountains of the USA. It is also shown that a composition of the river basin parent (surficial) rocks does not play a significant role in the variability of the r(rb)/r(bas) at this scale of the study. A comparison of the r(rb)/r(bas)-estimates and their factor dependence on the US rivers with the rivers of Northern Eurasia (the territory of the former Soviet Union) makes it possible to reveal good convergence of the results obtained in these parts of the Earth, and to suggest the universal nature of the revealed regularities (in total for 684 river basins) for the whole temperate (partly for subtropical and tropical) zone of the Northern hemisphere of our planet.