The eastern rim region of Hellas basin is characterized by the four prominent and quite extensively researched (cf. Crown et al., 2005, and references therein) outflow channels. In this work we focus on the Reull Vallis. On the basis of observations from available data sets, we present a hypothesis for the evolution of Reull Vallis and its complimentary fluvial system. We suggest that this system consists of parts that were formed during several phases rather than being a single continuous channel. Our results show that the fluvial system of Reull Vallis consists of two main parts and likely had independent formation phases and different sources of water. Our results also show that the upper portion of the Reull Vallis was formed by outflow from beneath Hesperia Planum (as proposed already in earlier works), but the suggested segments 1 and 2 (Mest and Crown, 2001) of the Vallis are not directly linked. There seems to have been an on-surface source for the formation of the segment 2 in the form of a topographic depression that was filled before the subsequent draining and formation of segment 2. Our interpretation of the evolution and formation implies a complex history for the Reull Vallis system.