Running surface waters, rivers and streams, differ from lotic (stagnant) waters in many characteristics. The main distinction focuses on relative residence times of the water; running waters have a much shorter retention time compared to lakes, reservoirs and oceans. Relatively long retention time means that pollutants will be present in the ecosystem for a longer period of time. Consequently, natural and anthropogenic watershed influences can have profound effects on water quality for human use and aquatic communities living within the ecosystem. This book presents a case study of anthropogenic impact on water quality of a reservoir located in western Slovenia (EU) and on its drainage area - its inflows from surrounding villages and agricultural land. Buffer capacity of natural wetlands was also studied. The presented results reflect the complexity of processes that occur in natural environment. The work highlights the importance of including the drainage area in water quality monitoring design. It contains some theoretical background about lakes, reservoirs, water quality, and wetlands and should be useful especially to students, professionals, and individuals new in the field.