GRAVITY, RADAR AND SEISMIC INVESTIGATIONS TO HELP DETERMINE GEOLOGIC, HYDROLOGIC, AND BIOLOGIC RELATIONS IN THE NYACK VALLEY, NORTHWESTERN MONTANA

Our interdisciplinary research on the intermontane Nyack floodplain of the Flathead River in northwestern Montana focuses on the relationships between physical and biological processes linking water, nutrients, and the evolution of the floodplain. As part of that research we are determining the configuration of the valley fill and its internal stratification. 153 new observations of the complete Bouguer gravity anomaly indicate a maximum depth to bedrock of about 150 meters in the Nyack Valley. However, the shape is not simple. Initial interpretation indicates at least two deeper sub-basins separated by a bedrock saddle about 500 meters wide. This saddle occurs in an area where warmer groundwater upwells into the river and surrounding area. Thus groundwater flow also indicates a bedrock high. The gravity data also indicate a package of Tertiary bedrock limited to the north eastern section of the valley. The signal from 79 lines (about 12 km) of Ground Penetrating Radar (GPR) data, collected with a 50 mhz antenna system, did not return information from the bedrock. However, the GPR data do indicate a reasonably consistent layer of coarse quaternary alluvium at approximately 30 meters depth. Seismic refraction results, from a 24-channel instrument, also indicate a stratigraphic boundary at about 30 meters (roughly 1400 m/s over 2100 m/s), as well as bedrock at about 35 meters where the gravity data and groundwater flow indicate a shallow saddle.