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Fifty-six riverbed surface sediment (RSS) samples were collected along the Ningxia-Inner Mongolian reaches of the Yellow River (NIMYR). These samples were analyzed to determine their heavy metal concentrations (Co, Cr, Ni, Cu, V and Zn), grain sizes, sediment sources and the causes of their heavy metal contamination. The cumulative distribution functions of the heavy metals in RSS of these reaches are plotted to identify the geochemical baseline level (GBL) of each element and determine the average background concentration of each heavy metal. Principal component analysis and hierarchical cluster analysis are conducted based on the grain sizes of RSS, and the samples are classified into two groups: coarse grained samples (CGS) and fine grained samples (FGS). The degree of heavy metal contamination for each sample is identified by its enrichment factor (EF). The results reveal that the coarse particle component (medium sand and coarse sand) in the bed materials is chiefly from the bordering deserts along the Yellow River. The clay and silt in the bed materials chiefly originate from the upper reaches of the Yellow River, and the fine sand is identified as a hybrid sediment derived from the upper reaches of the Yellow River and the bordering deserts. The CGS primarily appear in the reaches bordering deserts, and the sites are near the confluence of gullies and the Yellow River. The FGS are located adjacent to cities with especially strong industrial activity such as Wuhai, Bayan Nur, Baotou and Togtoh. The Cr, Ni, Cu, V and Zn concentrations (mg kg(-1)) are 84.34 +/- 49.46, 30.21 +/- 7.90, 25.01 +/- 7.61, 73.17 +/- 18.92 and 55.62 +/- 18.93 in the FGS and 65.07 +/- 19.51, 23.86 +/- 6.84, 18.04 +/- 3.8, 53.47 +/- 10.57 and 34.89 +/- 9.19 in the CGS respectively, and the concentrations of Co in the CGS (213.40 +/- 69.71) are notably higher than in the FGS (112.02 +/- 48.87) and greater than the Co GBL (210). The most contaminated samples in the NIMYR are adjacent to the cities of Wuhai (EFCr = 5.19; EFNi = 1.96), Bayan Nur (EFCr = 5.88; EFNi = 2.08) and Baotou (EFCu = 1.55; EFZn = 1.68) where the Cr, Ni, Cu, V and Zn concentrations are above the correlated GBLs (85, 34, 27, 75 and 62 mg kg(-1), respectively), which are mostly affected by industrial processes, and samples that are only moderately contaminated by heavy metals are found in the reaches bordering desert (Wuhai-Baotou) because contaminated sediments are diluted by uncontaminated desert sand. In contrast, all of the Cu, Cr, Ni, V and Zn concentrations in RSS of the Qingtongxia-Wuhai reach are lower than the correlated GBLs of elements. (c) 2015 Elsevier Ltd. All rights reserved.