Groundwater/Surface Water Interactions in the Upper Sammamish River:

Due to excessive temperatures, three segments of the Sammamish River are included on the federal Clean Water Act 303(d) list for Washington State impaired waters. Mini-piezometers were installed at nine locations along the upper six miles of the river in August 2001 to observe groundwater/surface water interactions during the fall low-flow period. Seven-foot long mini-piezometers were installed in the streambed to a depth of four to five feet below the sediment-water interface with openings in the bottom six inches. The difference between the water height inside and outside the piezometer was used as an indicator of flow direction in the immediate area. Measurements collected monthly in the fall of 2001 indicate that groundwater discharged to the river at eight of nine sites on all but one date. The flow direction at the site near Leary Way was consistently from the river to the streambed, contrary to geologic indications of discharge from the adjacent Bear Creek Alluvial Valley. The groundwater flow direction at the Marymoor Park site appeared to reverse slightly in November 2001, from discharging to the river in previous months to recharging the aquifer. Specific conductivity generally increased from upstream to downstream sites, indicating net discharge of higher conductivity groundwater along the study reach