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This article discusses the environmental issues facing small hydropower plants (SHPs) operating in temperate lowland rivers of Lithuania. The research subjects are two medium head reservoir type hydro schemes considered within a context of the global fleet of SHPs in the country. This research considers general abiotic indicators (flow, level, water retention time in the reservoirs) of the stream that may affect the aquatic systems. The main idea was to test whether the hydrologic regime has been altered by small hydropower dams. The analysis of changes in abiotic indicators is a complex process, including both pre-and post-reservoir construction and post commissioning of the SHPs under operation. Downstream hydrograph (flow and stage) ramping is also an issue for operating SHPs that can result in temporary rapid changes in flow and consequently negatively impact aquatic resources. This ramping has been quantitatively evaluated. To avoid the risk of excessive flow ramping, the types of turbines available were evaluated and the most suitable types for the natural river flow regime were identified. The results of this study are to allow for new hydro schemes or upgrades to use water resources in a more sustainable way.

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Small hydropower plants (installed power below 10 MW) are generally considered less impacting than larger plants, and this has stimulated their rapid spread, with a developing potential that is not exhausted yet. However, since they can cause environmental impacts, especially in case of cascade installations, there is the need to operate them in a more sustainable way, e.g. considering ecosystem needs and by developing low-impacting technologies. In this paper, an assessment was conducted to estimate how the environmental flow and the plant spatial density affect the small hydropower potential (considering run-of-river schemes, diversion type, DROR) in the European Union. The potential of DROR is 79 TWh/y under the strictest environmental constraints considered, and 1,710 TWh/y under the laxest constraints. The potential of low-impacting micro technologies (< 100 kW) was also assessed, showing that the economic potential of hydrokinetic turbines in rivers is 1.2 TWh/y, that of water wheels in old mills is 1.6 TWh/y, and the hydropower potential of water and wastewater networks is 3.1 TWh/y, at an average investment cost of 5,000 €/kW.