The McMurdo Dry Valleys of Antarctica contain many glacial melt water streams which connect glaciers, soils, and lakes. Although flow occurs in the austral summers for only 6 to 12 weeks, these streams are important landscape features influencing the response of the lakes to changing climate. Some of these streams have thriving mat communities composed of cyanobacteria. The mats are dormant when there is no flow in the streams and begin photosynthesizing with the onset of flow. The streams are composed of an open channel and hyporheic zone, which is visible as a wetted area adjacent to the stream. The hyporheic zone becomes saturated as the flow advances downstream and can correspond to a substantial storage zone for meltwater. The lower boundary of the hyporheic zone is controlled by thawing of the active layer during the summer. Tracer injection experiments show that the porosity of the unconsolidated alluvium results in rapid hyporheic exchange in dry valley streams. Monitoring of tracer arrival in hyporheic zone wells shows substantial heterogeneity in flowpaths. The hyporheic zone is also a zone of active weathering reactions, and thus hyporheic exchange strongly influences stream chemistry and ecology.