SHALLOW SUBSURFACE IMAGING WITH GROUND PENETRATING RADAR OF THE NYACK FLOODPLAIN, MONTANA

The Middle Fork of the Flathead River is the location of interdisciplinary research that focuses on the dynamic interaction between physical and biological processes that link water and materials flux and retention to fluvial landscape evolution. Zones of preferential flow serve as points of injection and drain for the alluvial aquifer and provide insect access to breeding grounds away from the river. Further, these preferential flowpaths are zones of highly diverse food webs that facilitate biogeochemical cycling within the floodplain. Previous work in the region detected stoneflies in wells that are up to 2 km away from the river.

To image shallow subsurface stratigraphy and morphology of the Nyack floodplain, we collected 20 km of data using 100 and 200 MHz GPR with standard filtering procedures. Given the cut and fill nature of the Middle Fork of the Flathead River, we expect to find discontinuous stratigraphy with a minimal amount of preserved paleochannels.

Filtered radargrams consist of chaotic, discontinuous reflection patterns mixed with sporadic zones of absorption. Four locations within the floodplain have trough-shaped reflectors indicative of paleochannels. These reflectors are evident in one percent of the data collected. Diffraction hyperbolae due to boulders are present throughout the floodplain at depths between three and twelve meters. Their depths indicate that deposition took place before the paleochannels and their distribution indicates the valley becomes deeper trending from the northeast to the southwest.