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Despite the fundamental role of river flow in determining the structure and function of lotic ecosystems, few studies have directly related features of the natural flow regime to variation in stream invertebrate assemblage composition. We classified 240 near-pristine, snowmelt-dominated Finnish streams into hydrological river types. We assessed the relationship of these types with benthic macroinvertebrate assemblage structure and examined the relative importance of hydrological variables, local-habitat variables, and geographical location in predicting variation in assemblage structure. We used a hydrological model to obtain site-specific daily discharges for a 30-y period (1981-2010) and calculated 223 flow indices based on flow index modeling tools. We used a combination of principal component and cluster analysis to classify the sites into 6 distinct hydrological types that were separated mainly by geographical location and catchment-size-related factors. Nonmetric multidimensional scaling and multiresponse permutation procedure showed that macroinvertebrate assemblage structure differed significantly among the hydrological types, but the associated A-statistic indicated wide variation among sites within type. Redundancy analysis indicated that assemblage structure was related more closely to hydrological variables than to localhabitat and spatial variables. The role of hydrology was confirmed further by functional trait structure that showed close relationships to hydrological variables in three-table ordination (RLQ) and fourth-corner analyses. Traits representing organism size and microhabitat preference were correlated most closely with hydrological variables describing variability or seasonality of flows. Our study highlights the role of hydrology in structuring stream assemblages in seasonal, snowmelt-dominated river systems. Modeling hydrological variables in combination with species distribution models may provide a tool for predicting future changes in species distributions in stream ecosystems. [ABSTRACT FROM AUTHOR]