Assessment of the Alluvial Sediments in the Big Thompson River Valley, Colorado

To obtain subsurface geologic information about the alluvium in the Big
Thompson River valley, S -wave refraction data were collected along three roads
that cross the valley. The traveltimes were processed to estimate velocities
and thicknesses for a layered-earth model; from these models, three cross
sections of the river valley were constructed. The river valleys are covered by
a layer of soil, which is 0.2 to 1.5 m thick. Beneath the soil, there is one
layer of alluvium at some locations and two layers at other locations. For the
two westernmost cross sections, the total thickness of the alluvium ranges from
about 6 to 10 m near the center of the valley and from about 2 to 6 m near the
sides of the valley. The easternmost cross section is somewhat more complex
than the other two, because it is near the confluence of the Big Thompson and
the Little Thompson Rivers. In this cross section, the thickness of the
alluvium ranges from about 8 to 10 m in the southern half of the valley and
from about 3 to 13 m in the northern half. In all three cross sections, the
alluvium overlies bedrock, which is the upper transition member of the Pierre
Shale.

[Summary provided by the USGS.]