Freshwater resources are under severe threat by anthropogenic pollution all around the world. There is a dire need for proper environmental monitoring and assessment for future planning and management of these assets. The present investigation was carried out in order to evaluate the distribution, correlation, source identification, bioaccessibility, health/ecological risk assessment, seasonal and spatial variations of selected metals (As, Ca, Cd, Co, Cr, Cu, Hg, Fe, K, Li, Mg, Mn, Na, Ni, Pb, Se, Sr and Zn) in the water, surface sediments, soil and selected fish species from/around Mangla Lake, Pakistan. The samples were collected during summer, winter, pre-monsoon, monsoon and post-monsoon seasons. In addition, various physicochemical parameters, such as, pH, Temperature (T), Dissolved oxygen (DO), Total alkalinity (TA), Electrical Conductivity (EC), Total Dissolved Solids (TDS), Total Hardness (TH), Bicarbonate ion (HCO3 -), Residual Sodium Bicarbonate (RSBC), Sodium Adsorption Ratio (SAR), Percent Sodium (PS), Kelly’s Ratio (KR), Magnesium Calcium Ratio (Mg/Ca), Magnesium Adsorption Ratio (MAR), Permeability Index (PI) in the water and/or sediments/soil were also estimated. Water samples were also analysed for selected anions (fluoride, chloride, nitrate and sulphate), while soil/sediments were assessed for their particle size distribution and texture. In case of soil/sediments, chemical fractionation of selected metals was carried out by sequential extraction, employing modified Community Bureau of Reference (mBCR) procedure. Bioavailability of the metals was assessed by using calcium nitrate extraction. Measurement of the metal levels in water, sediments, soil and fish samples was accomplished by flame atomic absorption spectrophotometer and inductively coupled plasma mass spectrometer under optimum analytical conditions. Correlation analysis was used to identify the mutual viable relationships among the metals and multivariate analysis including principal component analysis (PCA) and cluster analysis (CA) were used to categorize the pollution sources. Pollution indices, including enrichment factor (EF), geoaccumulation index (Igeo), contamination factor (Cf), degree of contamination (Cdeg), potential ecological risk factor (Ei), potential ecological risk index (RI), potential acute toxicity (TU), individual contamination factor (ICF), global contamination factor (GCF), risk assessment code (RAC) and heavy metal pollution index (HEI) were calculated to appraise the ecological risks associated with the pollutants in freshwater reservoir. Water quality of the freshwater reservoir for drinking and irrigation purposes was also assessed. Health risk assessment including exposure assessment, margin of exposure, hazard quotients, estimated daily/weekly intakes, non-carcinogenic and carcinogenic risk assessment (via ingestion and dermal exposure) was calculated to evaluate the adverse health effects on the exposed population. The pH, EC, TDS and Cl- levels in the water samples were within acceptable limits, while DO contents were comparatively lower, manifesting reducing conditions in the reservoir. Among the metals, dominant contributions were found for Ca, K, Mg and Na, while As, Li, Se, and Mn showed relatively lower contributions in the water samples. Most of the metals in water samples exhibited random distribution. Strong metal-to-metal correlations were noted among various metal pairs in water samples which indicated their communal variations/contributions. Mean levels of As, Cd, Co, Cr, Ni and Pb were higher than the national/international water quality guidelines, thus emerging as potential pollutants in the reservoir. PCA and CA evidenced largely anthropogenic contributions for As, Cd, Co, Cr, Ni and Pb in the water reservoir, which was found to be heavily polluted by As, Cd, Co, Ni, Cr and Pb. Water samples were observed to be unsuitable for irrigation with respect to bicarbonate and RSBC levels in all seasons, however, on the basis of SAR, xxxv KR, PS, MAR and Mg/Ca, water was suitable and safe for irrigation. Similarly, IWQ index exhibited high to medium suitability of water for irrigation purpose. Heavy metal pollution index (HEI) and degree of contamination (Cd) suggested medium degree of pollution in the water. HQing and HI values for As, Cd, Co, Cr, Ni and Pb were higher than safe limit (unity) indicating non-carcinogenic risks to the local population. The sediments and soil samples revealed comparatively higher concentrations for Ca, Mg, Fe, Na, Mn and K, while lowest concentrations were noted for Hg, Se, Cd and Li. Electrical conductivity (EC) and TDS showed large variations in the sediments/soil samples. Generally, selected metal concentrations in the sediment and soil samples exhibited random distribution as evidenced by higher skewness, SD and SE values. Strong and significant correlations were noted among various metals in the soil and sediment samples thus manifesting their common variations/origin. Concentrations of most of the metals in sediments/soil were significantly higher at sampling sites located near highly urbanized/semi-urban areas. PCA and CA showed anthropogenic contributions of As, Cd, Cr, Co, Hg, Ni, Pb and Se in the sediment and soil samples. Particle size analysis exhibited sandy loam texture of the soil/sediments. Enrichment factor, geoaccumulation index and contamination factor showed that among selected metals, As, Cd, Co, Hg, Pb and Se were highly enriched and accumulated in the sediments and soil samples. Potential acute toxicity indicated by toxic units (TUs) of the metals revealed that among the selected metals, As, Cd, and Hg showed relatively higher potential toxicity in the sediment/soil samples. Potential ecological risk index indicated very high risk in the sediment/soil samples. Pollution load index, degree of contamination and potential ecological risk index indicated more contamination during pre-monsoon than other seasons. Sediment quality guidelines (SQGs) indicated that As, Cd, Hg, Ni and Pb are likely to result in frequently occurring harmful effects on sediment-dwelling biota. Health risk assessment of the metals via ingestion/dermal route in sediments/soil showed little/no risk. Sediment and soil samples were found to be heavily/significantly polluted by As, Cd, Cr, Co, Hg, Ni, Pb and Se. Overall, As, Cd, Co, Cr, Hg, Ni, Pb and Se were primarily contributed by anthropogenic intrusions such as, agricultural activities, industrial activities, fuel combustion, atmospheric deposition and recreational activities and Ca, Cu, Fe, Li, K, Mg, Mn, Na, Sr and Zn were mainly contributed by natural inputs in the reservoir. Concentrations of selected metals were also measured in the muscles, gills and scales of fish species (Cyprinus carpio, Catla catla, Hypophthalmichthys molitrix, Wallago attu and Sperata sarwari) collected from the water reservoir. Generally, fish species exhibited higher concentrations of Ca, Fe, K, Mg, Na and Zn, while lowest concentrations were observed for As, Cd, Hg and Li. Highest concentrations of most of the metals were found in the gills, followed by scales and muscles. Most of the metal levels in the fish samples exhibited symmetric and normal distribution, except Ca, K, Na and Zn which showed predominantly non-Gaussian distribution. PCA and CA evidenced anthropogenic contributions of As, Cd, Co, Cr, Hg, Ni and Pb in the fish muscles. EWI and EDI values of As, Cd, Cr, Hg, Ni and Pb were noted to be higher than the recommended PTWI and PTDI values, respectively. Higher MOE values for As, Cd, Cr, Hg, Ni, Pb and Se in the muscles specified higher dose exposure, which may result in chronic non-carcinogenic effects to the consumers. HQing revealed that contaminated fish consumption may cause non-carcinogenic risks associated with the elevated levels of As, Cd, Cr, Hg, Ni, Pb, Se and Zn. Target cancer risk (TCR) demonstrated that the consumption of fish from Mangla Lake on continuous basis can result in exposure of elevated contents of As, Cd, Cr, Ni and Pb with a significant probability of lifetime carcinogenic risk to the consumers.