Several years of weekly sampling of waters from the Shinfa River watershed in the lowlands of northwestern Ethiopia yielded 275 samples with δD_vsmow and δ¹⁸O_vsmow values ranging from c. −10 to +100‰ and from c. −2 to +20‰, respectively. Wet season (summertime) Shinfa River water stable hydrogen and oxygen isotope values are among the lowest reported in this study, whereas the dry season (winter/spring) usually records a progressive trend towards +100 and +20‰, respectively. Overlapping with this interval of Shinfa River water sampling, air temperatures (n = 155) also were recorded at the same time; temperatures range from c. 18 to 47°C. The coolest temperatures occur during the summer wet season, associated with the arrival of the Kiremt rains in the region, whereas the warmest temperatures occur towards the end of the dry season. In order to evaluate the extent to which this rather extreme isotope hydrology is recorded in the sediments and biota of the Shinfa River system, both hardwater calcareous deposits precipitated on basalt cobbles by evaporation in the Shinfa River channel during the dry season and aragonite from three different modern bivalve mollusc species were collected and analysed for their stable oxygen and carbon isotope compositions. Hardwater calcareous deposit δ¹⁸O_vpdb and δ¹³C_vpdb values range from c. −2 to +5‰ and c. −9 to +7‰, respectively, and preserve a trend towards progressively more positive δ¹⁸O_vpdb and δ¹³C_vpdb values through the course of the dry season. Shinfa River mollusc aragonite powders (n = 51) were serially sampled from cf. Coelutura aegyptica, cf. Chambardia rubens and Etheria elliptica species. All species record oxygen and carbon isotopes between c. −2 and +7‰ and between c. −18 and −8‰, and each species records coherent trends between those extremes as well as a positive parametric correlation between measured oxygen and carbon isotope values. However, there does appear to be some variability of measured isotope values by species, suggesting that species-specific metabolic differences may impact the resulting range of aragonite stable carbon and oxygen values. Based upon the measured Shinfa River water δ¹⁸O_vsmow and corresponding water temperatures at the time of sampling, a possible range of Shinfa River calcite and aragonite δ¹⁸O_vpdb values were calculated in conjunction with well-established calcite–water and aragonite–water oxygen isotope fractionation equations. These ‘fictive’ calcite and aragonite δ¹⁸O_vpdb values range from c. −5 to +15‰, which is a much larger range than previously documented from analyses of the hardwater calcareous deposits and mollusc aragonite samples. The narrower range of values in the natural calcite and aragonite samples may be attributed to several mechanisms, including time averaging and environmental stress. Nevertheless, the stable oxygen isotopic compositions of these natural samples offer a minimum assessment of the environmental extremes which occur in this region today, and provide a model for reconstructing the environments of the past.