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Spatiotemporal variations of dissolved organic carbon (DOC) and inorganic carbon (DIC) in 26 waters across the semi-humid/semi-arid Songnen Plain, China, were examined with data collected during 2008–2011. Fresh (<i>n</i> = 14) and brackish (<i>n</i> = 12) waters were grouped according to electrical conductivity (threshold = 1000 μS cm^&minus;1) Significant differences in the average DOC and DIC concentrations were observed between the fresh (5.63 mg L⁻¹, 37.39 mg L^&minus;1) and the brackish waters (15.33 mg L⁻¹, 142.93 mg L^&minus;1). Colored dissolved organic matter (CDOM) and DOC concentrations were mainly controlled by climatic–hydrologic conditions. The investigation indicated that the outflow conditions in the semi-arid region had condensed effects on the dissolved carbon, resulting in close relationships between salinity vs. DOC (<i>R</i>² = 0.66), and salinity vs. DIC (<i>R</i>² = 0.94). An independent data set collected in May 2012 also confirmed this finding (DOC: <i>R</i>² = 0.79, DIC: <i>R</i>² = 0.91), highlighting the potential of quantifying DOC and DIC via salinity measurements for waters dispersed in the plain. Indices based on the CDOM absorption spectra (e.g., the DOC-specific CDOM absorption (SUVA₂₅₄), absorption ratio <i>a</i>₂₅₀ : <i>a</i>₃₆₅ (<i>E</i>₂₅₀ : <i>E</i>₃₆₅) and the spectral slope ratio (Sr, <i>S</i>_{275&minus;295}/<i>S</i>_{350&minus;400}) were applied to characterize CDOM composition and quality. Our results indicate that high molecular weight CDOM fractions are more abundant in the fresh waters than the brackish waters.