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Geophysical constraints on the evolution of an ephemeral channel at the Sand Creek Massacre National Historic Site, Colorado, USA




Sparks, Brad, author
Harry, Dennis, advisor
Wohl, Ellen, committee member
Nelson, Peter, committee member

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A geophysical survey was conducted on an ephemeral channel, Sand Creek, at Sand Creek Massacre National Historic Site to test three hypothesized migration and depositional models of ephemeral streams. A key motivation for the study is to identify the historical location of Sand Creek, which is critical to establishing the location of the 1864 Sand Creek Massacre. Hammer seismic refraction data were collected on 7 valley-wide lines oriented perpendicular to the channel, and ground penetrating radar data (200 MHz antenna) was collected on a grid overlying the channel and the channel banks. An additional GPR line (100 MHz) was collected on a line spanning the valley bottom. The refraction data show 4 layers: an eolian cap that is 1 - 3 m thick with a velocity of 0.3 km/s; a gradational alluvium layer consisting of ephemeral deposits which is 2 – 3 m thick with velocities ranging from 0.5 - 1 km/s; a gradational alluvium layer consisting of perennial fluvial deposits which is 2 - 7 m thick with velocities ranging from 1.2 - 2.9 km/s; and a homogeneous layer with a velocity of 2.4 km/s which is interpreted to be the Pierre Shale Formation. The radar data located buried channel boundaries and revealed a change in bedforms at 3 - 4 m deep. The change in bedforms is interpreted to indicate a flow regime change from an older perennial to a more recent ephemeral flow. The channel bedforms within the ephemeral flow regime deposits suggest that the channel has not migrated across the modern valley since the ephemeral flow regime was established, but punctuated changes in morphology within the channel have occurred in association with major floods. The results indicate that the channel has not changed position in historical times. This suggests that the modern stream is the proper geographic context for historical accounts that reference the location of Sand Creek when describing events that occurred during the 1864 massacre.


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