Siltation changes the limnological properties of water and is the most challenging of all reservoir problems. It impairs water quality, quantity and fauna assemblages, thereby eroding the functions of a reservoir. Siltation in Agba reservoir, Ilorin, Nigeria, was assessed using various physicochemical and biotic parameters. There was significant reduction (p < .05) in depth compared with original depth of the reservoir. Transparency was low, and turbidity was high especially during the rainy season; pH was slightly alkaline and water hardness is soft. Alkalinity ranged between 88 and 90 mg/L, nitrate and nitrite were low, and sulphate was high. Dissolved oxygen (DO) and biological oxygen demand (BOD) varied between 4.73–5.08 mg/L and 2.25–2.40 mg/L, while electrical conductivity and total dissolved solids ranged between 62.19and 62.72 µS/cm and 41.68 and 42.02 mg/L, respectively. Significant (p < .05) negative correlations were recorded between turbidity and transparency, turbidity and DO, while significant (p < .05) positive correlations were found between turbidity and BOD, turbidity and sulphate. Phytoplankton, zooplankton and fish assemblages were few with only 12 genera of phytoplankton, 10 genera of zooplankton and six species of fish recorded. Siltation of the reservoir was due to anthropogenic activities in the watershed, which gave rise to high turbidity, low transparency, reduction in depth and storage capacity. Low species assemblages of phytoplankton, zooplankton and fish were due to siltation in the reservoir. Siltation is gradually eroding the aesthetic value, tourism, drinking and fisheries functions of the reservoir and in a few years, if not controlled, the reservoir could be turned into a marsh. Siltation in the reservoir can be controlled through watershed management by soil erosion control, stopping anthropogenic activities which bring in sediments, bypassing the reservoir to create sediment basin or dike, sluicing, dredging, flushing, excavation and hydrosuction. If all these management fail, it will be appropriate to decommission the reservoir.