The study determines the physico-chemical parameters of water from Odo-Ebo River with a view to providing information on the effects of discharge from carwash on the River’s water quality. Samples (48) for physico-chemical water quality were collected monthly over a period of an annual cycle (February 2015–January 2016) from four stations, namely: upstream (about 500 m before the carwash), two discharge points (1st and 2nd discharge points), and downstream (about 500 m after the carwash). The discharge points had higher mean values in 9 out of the 13 investigated water quality parameters, especially plant nutrient ions (phosphate, nitrate, and sulphate), with the discharge points significantly differing from the other stations in terms of phosphate concentration and apparent colour. This led to reduction in dissolved oxygen and zooplankton abundance, changes in apparent water colour, and increase in COD at the effluent receiving points. Furthermore, the higher mean values of dissolved oxygen, BOD5, turbidity, and true colour recorded upstream and downstream were reflections of higher biological productivity and organic detritus at these stations as compared to discharge points. The specific presence of some eutrophic species at the discharge points has only further proven the negative effects the effluent from the carwash had on the river, thus posing a potential threat to its aquatic organisms’ diversity. This eutrophication effect was observed downstream as an increase in zooplankton abundance and diversity. Therefore, environmental protection regulations are needed to reduce anthropogenic influence on the rivers in Nigeria.