Records of δ18O in stream flow are critical for understanding and modeling hydrological, ecological, biogeochemical and atmospheric processes. However, the number of such records are extremely limited globally and the length of such time series are usually less than a decade. This situation severely handicaps their use in model testing and evaluation. Here we present a global assessment of freshwater mollusk (bivalves & gastropods) isotope data from 25 river basins that have stream water isotope values, water temperature data and shell material isotope signatures. Our data span a latitude range of 37.50°S to 52.06°N. We show that δ18O signatures in freshwater mollusks are able to explain 95% of the variance of stream water δ18O. We use shell δ18O values and water temperature data to reconstruct stream water δ18O signatures. With freshwater mussel life expectancy ranging from a few years up to 200 years, this translation of mollusk metabolic properties into long term stream water isotope records is a promising approach for substantially extending global stream water isotope records in time and space.

For several decades, stable isotopes have been a commonly used and effective tool for flow path analysis, stream water source apportionment, and transit time analysis. The Global Network of Isotopes in Precipitation repository now has monthly precipitation isotope time series extending over several years and even decades in some settings. However, stream water isotope composition time series remain rather short with only very few data sets spanning over more than a few years. A critical challenge in this respect is the collection of stream water isotope data sets across a wide variety of headwater streams and for long durations. We rely on a new approach for stream signal reconstruction based on freshwater mussels, specifically the freshwater pearl mussel Margaritifera margaritifera. We use secondary ion mass spectrometry (SIMS) to quantify oxygen isotope ratios in pearl mussel shell growth bands. In our study area, the observed seasonal variability in precipitation δ18O values ranges between −15‰ and −3‰. This input signal is strongly damped in stream water, where observed values of δ18O range between −10‰ and −6.5‰. These values are consistent with our measured average shell‐derived stream water δ18O of −7.19‰. Along successive growth bands, SIMS‐based stream water δ18Ow values varied within a seasonal range of −9‰ to −5‰. The proposed SIMS‐based shell analysis technique is obviously well suited for analysing isotopic signatures of O in shell material—especially from the perspective of reconstructing historical series of in‐stream isotope signatures.