Kasei Valles (fig. 1) make up the largest system of outflow channels on Mars and were a major contributor of water to Chryse Planitia. The walls and floors of the Kasei channels are terraced and grooved, closely resembling the channeled scablands of eastern Washington State that were formed by catastrophic floods probably lasting no more than a few days (Baker and Milton, 1974; Baker and Kochel, 1979). Evidence obtained from previous geologic mapping of parts of Kasei Valles (Chapman and Scott, 1989) was not conclusive as to whether water levels varied markedly during single flood and erosional event or whether flooding was episodic and marked by intermittent periods of scouring. This problem – whether one or several flood episodes occurred within individual water courses – has been a continuing issue in studies of Martian channel formation (Greeley and others, 1977). Recent large-scale geologic mapping (Tanaka and Chapman, 1990) of Mangala Valles, another large outflow channel system in the Memnonia region of Mars, shows deposits of two periods of flooding; the deposits are separated stratigraphically by a lava flow. In areas around the Chryse basin, geologic studies (for example, Greeley and others, 1977) indicate that more than one episode of channel formation occurred or, less likely, that flooding was of very long duration. Evidence disclosed by the present mapping indicates that flooding was episodic in Kasei Valles and probably occurred over protracted time intervals throughout the Hesperian Periods and possibly in the Early to Middle Amazonian.