High-frequency water quality measurements in streams and rivers have expanded in scope and sophistication during the last two decades. Existing technology allows in situ automated measurements of water quality constituents, including both solutes and particulates, at unprecedented frequencies from seconds to subdaily sampling intervals. This detailed chemical information can be combined with measurements of hydrological and biogeochemical processes, bringing new insights into the sources, transport pathways, and transformation processes of solutes and particulates in complex catchments and along the aquatic continuum. Here, we summarize established and emerging high-frequency water quality technologies, outline key high-frequency hydrochemical data sets, and review scientific advances in key focus areas enabled by the rapid development of high-frequency water quality measurements in streams and rivers. Finally, we discuss future directions and challenges for using high-frequency water quality measurements to bridge scientific and management gaps by promoting a holistic understanding of freshwater systems and catchment status, health, and function.

River regulation by dams can alter flow regimes and organic matter dynamics, but less is known about how unregulated tributaries regulate organic matter composition and processing in the regulated river below the confluence. This study reports on water chemistry, especially dissolved organic matter (DOM) concentration and composition (dissolved organic carbon (DOC), organic nitrogen (DON), organic phosphorus (DOP) and combined amino acids (DCAA)) along the regulated Tumut and unregulated Goobarragandra (tributary) rivers under different flow conditions (base flow vs storm event) in south-east Australia. The tributary was significantly different from regulated and downstream sites during base flow conditions with higher temperature, pH, buffering capacity, DOC and nutrient concentrations (DON, DOP, DCAA). DOM characterisation by spectrometry and size exclusion chromatography revealed that the tributary contained a higher proportion of terrestrially derived humic-like and fulvic-like DOM. In contrast, regulated and downstream sites contained higher proportion of microbially derived DOM such as low molecular weight neutrals and protein-like components. Storm pulses of tributary flows into the regulated system, influenced both concentration and composition of DOM at the downstream site, which more strongly resembled the tributary site than the regulated site during the storm event. Additionally, we found that the tributary supplied fresh DOM, including small organic molecules to the regulated system during storm events. The presence of these different types of labile DOM can increase primary productivity and ecological functioning within regulated river reaches downstream of tributary junctions. This has important implications for the protection of unregulated tributary inflows within regulated river basins.

Many catchments in Nepal are affected by intensive agricultural activities, leading to extensive pesticide usages. This study aimed to assess pesticide abundance in concurrently collected water, sediment and fish samples for the first time in intensively cultivated catchment (Indra Sarowar) located in the mid-hill region of Nepal during the rice and vegetables growing season. A total of 75 pesticides were analysed, of which 4 pesticides (alachlor, diuron, metalaxyl and pyrimethanil) were present in water with detection frequency (DF) > 40%, with alachlor (0.62 – 2.68 µg L −1 ) being ubiquitous. In the sediment of tributaries, the pesticides p,p′-DDT, β-HCH, alachlor and diuron were detected with DF exceeding 40%, where β-HCH was commonly observed (DF = 92%) with concentration ranging from 6.29 – 99.22 µg kg −1 . The ecotoxicological risk indicated that herbicides (alachlor and diuron) posed a high risk to aquatic organisms in both tributaries and reservoir water. Such risk in sediment was even more pronounced, with alachlor and diuron showing up to 2.3 and 53.7 times higher risk respectively compared to water samples. However, none of these herbicides were detected in fish muscles. Among the fish species studied, pyrimethanil was the only quantified pesticide in edible tissue of both cage cultured (0.35 – 1.80 µg g −1 ww) and open stock fishes (0.06 – 1.12 µg g −1 ww). The consumer risk assessment showed very low human health risk associated with fish consumption (HQ < 0.2). Nonetheless, long-term consumption of contaminated fish may pose some risk that cannot be ignored. Overall, this study generated the benchmark data highlighting pervasive presence of banned (DDT, endosulfan, HCH) and unapproved (alachlor, diuron, pyrimethanil) pesticides in the environmental compartments in the mid-hill’s streams of Nepal. Graphical Abstract