Although spatial scale, upstream–downstream gradients, environmental variability, and floodplain connectivity and complexity have been recognized as integral components of our understanding of riverine pattern and process, aquatic invertebrate community structure across lateral gradients from headwaters to the lowland of a river has received little empirical documentation. Aquatic invertebrate density, distribution, and richness were investigated at 29 study sites distributed laterally (main channel–floodplain) among 6 geomorphologically distinct river reaches from the headwaters to near the mouth of one of the last seminatural river corridors draining the European Alps, the Tagliamento River in northeastern Italy. Community relationships were examined using multivariate and diversity analyses to investigate patterns of taxonomic richness. Diversity parameters were scaled at the site, reach, and catchment levels to better understand local and regional patterns. Mean invertebrate density increased from 7484 ± 2480 (±1 SE) individuals/m² in the headwaters to 98,811 ± 18,037 individuals/m² on the lowland floodplain. Crustacea, Coleoptera, and Ephemeroptera richness increased and nonchironomid Diptera and Plecoptera richness declined along the corridor. Richness (α diversity) and turnover (β diversity) showed no consistent increase or decrease along connectivity gradients within geomorphic reaches along the river corridor, possibly because each reach had unique characteristics. α diversity was similar among geomorphic reaches along the corridor, whereas β diversity between reaches increased as a function of distance between them. Direct gradient analysis indicated high taxa-to-environment concordance along lateral (i.e., floodplain water bodies) and longitudinal (upstream–downstream) dimensions and indicated distinct floodplain communities. These results demonstrate how lateral gradients (floodplain dimension) structure zoobenthic communities and how this structure can change along the longitudinal dimension (upstream–downstream). Our results highlight the importance of off-channel environments to macroinvertebrate community structure and diversity. Conservation, restoration, and preservation measures must consider the lateral dimension along river corridors as integral to maintaining diversity and community structure.