Channel deposit orientation controls aquifer anisotropy and, thus, groundwater flow and contaminant transport directions, at the Lawrence Livermore National Laboratory (LLNL), located in the southeast corner of the Livermore Basin, California, USA.  Therefore, understanding the depositional history at the site is critical for predicting groundwater flow and contaminant transport at LLNL.  Two streams currently contribute sediments to the study area – the Arroyo Seco (AS) from the south and the Arroyo Las Positas (ALP) from the east.  Geomorphic and petrologic evidence exists for stream capture and piracy of AS in the basin, thus producing aquifer zones with varying anisotropy directions.  The geomorphic evidence for stream capture includes (1) AS currently flows north into the Livermore Basin, bisecting an up-thrown block created by the Las Positas Fault, (2) an abandoned geomorphic AS fan south of the up-thrown block, and (3) a modern AS fan north of the up-thrown block that forces ALP northward in the basin.  Stratigraphic evidence for the capture of AS resulted from a provenance study which was conducted using gravel units from 17 well cores.  The AS sediments (marked by presence of clasts derived from the Franciscan Formation) occur at depths below 35m, and are overlain by ALP sediments (containing significantly fewer Franciscan-derived clasts) between 20 and 35m depth, which are, in turn, overlain by AS sediments to the surface.  This layering of the sediments indicates the AS was kept from the basin by the up-thrown fault block, during which time the ALP sediments were deposited; the AS was then redirected into the basin by stream capture.  The stacked sediments from the northerly flowing AS (which trend N-S to NW-SE) and the westerly flowing ALP (which trend more E-W) cause apparent anisotropy in the sediments and resulting hydraulic conductivity at LLNL, which is observed in pumping test results.