Valley networks on Mars are the most obvious features attesting that different geologic processes and possibly climatic conditions existed in the past. THEMIS images reveal valley networks within Melas Chasma, in Valles Marineris, a Hesperian-age canyon system. The valley networks in Melas Chasma are dense and highly organized, and the heads of the valleys are scattered at different elevations. All these features suggest that the networks were fed by precipitation. The morphological details reveal inner channels on some valley floors, attesting that water flowed within these valleys. On the DEM, the valleys flow into a completely enclosed depression. The edge of this feature follows a MOLA contour line, and the depression shows many sedimentary morphologies suggesting lacustrine environment. These landforms are located on remnants of layered deposits possibly composed of sulfate layers suggesting that fluvial activity could have contributed to the erosion of the layered terrains in Valles Marineris. Collectively, the features in Melas Chasma are a maximum of Hesperian in age. These results suggest that warm, wet environmental conditions on Mars persisted through the Hesperian and were present during the formation of Valles Marineris. The evidence for a paleolake in Melas Chasma attests to adequate environmental conditions for life development through the Hesperian period.