Freshwater reservoir effects can be large and highly variable. I will present my investigations into the short-term variability of the freshwater reservoir effect in two Northern German rivers. The samples analysed in this study were collected between 2007 and 2012. Reservoir ages of water samples, aquatic plants and fish from the rivers Alster and Trave range between zero and about 3,000 radiocarbon years. The reservoir age of water DIC depends to a large extent on the origin of the water and is for example correlated with precipitation amounts. These short-term variations are smoothed out in water plants. Their carbon should represent an average value of the entire growth season. However, there are large reservoir age variations in aquatic plants and animals as well. These can best be explained by the multitude of carbon sources which can be utilized by aquatic organisms, and which have potentially very different radiocarbon ages. Finally, I will discuss the influence of bomb carbon on radiocarbon dating of modern freshwater samples.