Currently, the literature lacks comprehensive studies on the impact of hydropower plants (HPs) on the environment, including studies focused on the physicochemical parameters of water. The aim of the article is to verify the current state of knowledge on the impact of run-of-river HPs on 17 physicochemical parameters of water. The article is in line with the recommendations of the European Union that the member states, under the common energy policy, should increase the share of renewable energy sources in the energy and perform environmental impact assessments of such facilities. As a result of the analysis carried out on three HPs (Sadowice, Skałka and Marszowice) located on the Bystrzyca River (a tributary of the Odra River in Poland), it was found that HPs affect the selected physicochemical parameters of the water, i.e., (p < 0.05): electrolytic conductivity (EC; Skałka, Marszowice HPs), pH (Skałka, Marszowice HPs); nitrate nitrogen (NO3-N; Marszowice HP), dissolved oxygen (DO; Marszowice HP) and ammonium nitrogen (NH4-N; Marszowice HP). The largest (>5%), statistically significant mean cumulative effect below Marszowice HP concerned NH4-N (−27.83%), DO (+14.04%) and NO3-N (+5.50%). In addition, it was observed that the effect of HPs increases in direct proportion to the damming height, and that run-of-river HPs have a lesser impact on the physicochemical parameters’ values than in storage HPs. Our results were in accordance with those of other scientists in terms of the increase in DO, the decrease in EC, and the decrease in total phosphorus concentrations below HPs.