The Nyack floodplain of the Middle Fork of the Flathead River is an intermontane floodplain in northwest Montana. Nyack is the location of interdisciplinary research that focuses on the dynamic relationships between physical and biological processes that link flux and retention of materials to evolution of the fluvial landscape. Paleochannels are hypothesized zones of preferential flow that serve as injection and drain points for the aquifer. Furthermore, these paleochannels are localities of diverse above and below ground food webs that facilitate biogeochemical cycling. Unfortunately, paleochannel identification has been limited to observation of swales from aerial photographs, in the field observations, and a limited geophysical study.

To determine the presence of paleochannels, image shallow subsurface stratigraphy, and detect the water table of the Nyack floodplain, I collected twenty kilometers of data using high-frequency ground-penetrating radar (GPR). I applied standard post-processing procedures to the raw GPR data to improve visualization of the shallow subsurface.

Post-processed radargrams consisted of chaotic, discontinuous reflection patterns mixed with zones of absorption. I identified four locations within the Nyack floodplain where trough-shaped reflectors, indicative of paleochannels, are present. These trough-shaped reflectors are evident in one percent of the data collected. In addition, multiple diffraction hyperbolae are located throughout the floodplain at depths between three and twelve meters.

Imaged paleochannels are present underneath three swales at Nyack. However, the longitudinal extent of a swale does not correlate with the longitudinal extent of the associated paleochannel. Furthermore, there is no connectivity between the imaged paleochannels or with the Middle Fork of the Flathead River.

Diffraction hyperbolae are due to boulders deposited before the imaged paleochannels. These boulders are most likely the result of glacial deposition or fluvial lag deposits. The depths of the boulders show increase in depth as one traverses from the northeast to the southwest across the floodplain