<p>For freshwater supplies, the Kingdom of Saudi Arabia (KSA) relies on desalinated saltwater, untreated and treated groundwater, and surface water. Only limited regions of KSA, including the province of Al-Baha, have access to surface water. Al-Baha region’s primary supply of freshwater is surface water, which is stored in various dam reservoirs, such as Wadi Thurad reservoir. So, an assessment of the produced water quality of Wadi Thurad reservoir is essential. Physical, chemical and biological characteristics of raw water and treated water were collected (2021–2022). Descriptive statistical analysis for the collected data were done, including Whisker Box-plot graphs. Moreover, a water quality index (WQI) approach was developed according to the Saudi standards to assess the water treatment plant performance. The statistical evaluation of the Water Quality (WQ) data under consideration revealed that several raw water readings exceeded the Saudi standards. Free chlorine (mainly) and bacteria (limited) in the generated water, however, were higher than the accepted levels. Inflow water was given a “Poor” rating by the created WQI, whereas treated generated water received a “Fair” rating, reflecting the effectiveness of the water treatment plant (WTP). The developed water quality index of this study should be applied in all water treatment plants in KSA to control their performance. A water quality monitoring plan for the reservoirs in KSA should be unified and applied.</p> Graphical abstract <p></p>