There is a wide consensus among hydrologists that discharge during the flow recession consists mainly of baseflow, i.e., delayed runoff exfiltrating from the shallow groundwater reservoir and of interflow. It is scarcely noticed, however, that also during floods a considerable portion of discharge is baseflow. Therefore, recession and baseflow analyses do not only concern low flow and groundwater recharge questions but also floods. In this study, the nonlinear reservoir theory is used, assuming nonlinear relation between groundwater storage and baseflow discharge in the river and it is applied for baseflow separation and analysis. Case studies are performed for a time series of daily flows from Melen and Solakli watersheds in the flood-prone Western and Eastern Black Sea regions in Turkey. The case studies revealed the evident seasonal behavior of baseflow. The water balances showed that the groundwater recharge is higher than the baseflow discharge during the wet season when flows in the streams increase. The baseflow discharge became greater than the groundwater recharge during the months with decreasing flow towards the summer season. It can be concluded that groundwater recharge, outflow and evaporation losses depend highly on seasonality and flow magnitude in the Black Sea region in Turkey.