The aquatic ecosystems of Chukka Beach (Sharda River) and the Sharda Sagar Dam within the Pilibhit Tiger Reserve (PTR), Uttar Pradesh, represent ecologically important freshwater habitats of the Terai landscape. The present study (2023–2025) investigates the diversity, abundance, and bioindicator role of aquatic insects in assessing the water quality of these two contrasting aquatic sites. A total of 18 aquatic insect species belonging to major orders such as Ephemeroptera, Trichoptera, Odonata, Coleoptera, Hemiptera, Diptera and Megaloptera were documented. Standard sampling (sweep netting, kick sampling, hand picking) and physicochemical analyses were used to study the interaction between aquatic insect assemblages and water quality parameters. Diversity indices (Shannon–Wiener, Simpson’s Dominance, Margalef Richness) were calculated seasonally for both sites. Bioassessment indices including BMWP, ASPT, and Family Biotic Index (FBI) were applied to evaluate the ecological condition of the water bodies. Correlation analysis revealed strong associations between insect diversity and key environmental variables such as pH, indicating that slightly alkaline conditions support healthier insect communities. In contrast, fluoride, arsenic, BOD, COD, TDS, Water temperature and copper (Cu) showed negative correlations with species richness and abundance .But in sensitive taxa like Ephemeroptera and Trichoptera showed higher representation at Chukka Beach, indicating relatively better water quality, whereas tolerant groups such as Chironomidae, Culicidae, Ceratopogonidae, Psychodidae, Tabanidae (Diptera) were more abundant near the Sharda Sagar Dam influenced by anthropogenic activities. Canonical Correspondence Analysis (CCA) further illustrated how environmental gradients shape the community structure., suggesting Higher TDS and copper levels also contributed to declining diversity, indicating overall degradation of water quality. Correlation analysis showed that aquatic insect diversity positively correlated with pH because even slightly alkaline conditions favour healthy insect communities. On the other hand--Water temperature, pH, Nitrate, Total alkalinity, Dissolved oxygen (DO), Nitrate, TDS, BOD and COD identified correlations with the richness and abundance of species. Among them, arsenic, BOD and COD had the strongest negative effects, which implies that toxic chemicals and organic contamination reduce species richness of sensitive taxa. Higher TDS and copper content was also associated with diminished diversity and hence general deterioration of water quality. Altogether, the results provide supporting evidence for aquatic insects to be good bio-indicators of health in freshwater resource of Pilibhit area. The study is of significance as it offers an opportunity for baselines for conservation monitoring of the Sharda River system, and its conservation planning and sustainable management.