The flow exchange between open channel and alternate sandbar is important for instream ecosystems, water balance and water quality problems investigations. Both experimental and numerical investigations are done to get relationships between surface and subsurface flowin longitudinal and transverse direction. Under steady low flow condition, strong effects of surfaceflow conditions on subsurface flow structures in the alternate sandbars are recognized through experiments as:(1) Longitudinal flow is dominant, but the longitudinal distribution of subsurface water elevations depend on surface flow conditions.(2) Transverse gradient of subsurface flow varies by surface water discharge and bed shape. Three types of subsurface flow patterns are recognized to define flowexchange between stream and sandbar. Numerical simulations using 2-D theory shows good fit with the experimental results and field data.