In a confined alluvial aquifer located between two rivers, discrete zones of anomalously high concentrations of redox species such as iron, are thought to be a result of groundwater flow dynamics rather than a chemical evolution along continuous flow paths. This new hypothesis was confirmed at a study site located between Nan and Yom rivers in Phitsanulok, Thailand, by analyzing concentrations of redox species in comparison with dynamic groundwater flow patterns. River incision into the confined alluvial aquifer and seasonally varying river stages result in truncated flow paths. The groundwater flow dynamics between two rivers has four phases that are cyclic, including: aquifer discharge into both rivers, direct flow from one river toward another, aquifer recharge from both rivers, and reverse of river-to-river flow. The resulting groundwater flow direction has a zigzag pattern and its general trend is almost parallel to the river flow. High iron anomaly appears as discrete zones in the transition areas of the confined alluvial aquifer because the lateral recharge from rivers penetrates into the aquifer only by tens of meters. The high iron anomaly, which is nearly constant in space and time, is a result of groundwater/ surface-water interactions and related groundwater flow dynamics. © Springer-Verlag 2006.