Rivers suffer from multiple stressors acting simultaneously on their biota, but the consequences are poorly quantified at the global scale. We evaluated the biological condition of rivers globally, including the largest proportion of countries from the Global South published to date. We gathered macroinvertebrate- and fish-based assessments from 72,275 and 37,676 sites, respectively, from 64 study regions across six continents and 45 nations. Because assessments were based on differing methods, different systems were consolidated into a 3-class system: Good, Impaired, or Severely Impaired, following common guidelines. The proportion of sites in each class by study area was calculated and each region was assigned a Köppen-Geiger climate type, Human Footprint score (addressing landscape alterations), Human Development score (addressing social welfare), % rivers with good ambient water quality, % protected freshwater key biodiversity areas; and % of forest area net change rate. We found that 50% of macroinvertebrate sites and 42% of fish sites were in Good condition, whereas 21% and 29% were Severely Impaired, respectively. Poorest biological conditions occurred in Arid and Equatorial climates and the best conditions occurred in Snow climates. Severely Impaired conditions were associated (Pearson correlation coefficient) with higher Human Development Index scores, poorer physico-chemical water quality, and lower proportions of protected freshwater areas. Good biological conditions were associated with good water quality and increased forested areas. It is essential to implement statutory bioassessment programs in Asian, African and South American countries, and continue them in Oceania, Europe and North America. There is a need to invest in assessments based on fish, as there is less information globally and fish were strong indicators of degradation. Our study highlights a need to increase the extent and number of protected river catchments, preserve and restore natural forested areas in the catchments, treat wastewater discharges, and improve river connectivity.

Benthic macroinvertebrate and fish assessment data from national/continental validated databases, from peer reviewed publications, or from research projects with validated and/or published biological classification methods were collected by the authors from each country. We targeted the most recent bioassessment data available for each site. Data sources are indicated in the csv file. The biological quality indices used were those previously validated for the study regions and were based on the reference condition approach (RCA, Hughes et al. 1986, Reynoldson et al. 1997, Stoddard et al. 2006). This means that the index scores were adjusted to the reference values of the most appropriate river type/ecoregion. For multimetric indices and predictive models the index score produced is on a 0-1 scale. The use of the RCA and normalized scales contribute to reducing the importance of differing sampling methods and indices.To homogenize site assessments globally and reduce the importance of differences in study designs and sampling and assessment methods, we converted all the site quality classification systems into a simple 3-class system: Good (no rehabilitation measures needed, assemblages similar to reference conditions); Impaired (scores clearly lower than in reference conditions); and Severely Impaired (many fewer taxa than expected; substantial alteration in assemblage composition compared to reference conditions). In practical terms, this means that the High and Good classes from European indices, the Equivalent to Reference of BEAST predictive models, the A band of AUSRIVAS and RIVPACS models, or classes A (Pristine) and B (largely natural) of the South African system were considered as Good in our classification system. The remaining degradation gradient was divided into two classes. An intermediate degradation class (Impaired) corresponds with classes C (moderately modified) and D (largely modified) in the South African system, and to the Moderate and Poor classes of the WFD system. A substantial degradation level (Severely Impaired) integrated classes E (seriously modified) and F (Critically modified) from South Africa, and Bad from the WFD. Those results were reported as % of sites in Good, Impaired and Severely Impaired for each study region.

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