Analysis of anthropogenic and runoff erosion into sandstone canyon walls
using repeat structure-from-motion
Modern bedrock streams and rivers emerged in the Midwestern United
States in response to glacial outwash floods ~19,000
years ago and continued to adjust to new drainage patterns over the
landscape formed by Wisconsin glaciation events. The Illinois River and
corresponding tributaries form a fascinating landscape with canyons
carved 50-200 m deep into the St. Peter Sandstone. These geomorphic
features are preserved and open to visitors as Starved Rock State Park
in Central Illinois. Free access to the state parks and close proximity
to the large Chicago metropolitan area result in frequent visitation to
this natural attraction. Recently, the rates of natural stream incision
appear to be overprinted by direct human influences on the landscape.
The St. Peter Sandstone is a weakly cemented, extremely friable quartz
arenite, making it susceptible to human disturbance and rapid natural
erosion. This study explores how quickly present day changes occur along
bedrock surfaces using photogrammetric Structure-from-Motion (SfM).
Repeat photographic data were collected once per month from two sites
within the park where canyon walls contained human carvings that served
as reference features to align photos and monitor change. Photos were
merged in Agisoft PhotoScan Pro to construct 3D point clouds and
imported into CloudCompare to measure changes to rock surfaces between
monthly visits. The photogrammetric SfM analysis detected measurable
change on a centimeter to millimeter scale. Changes along footpaths were
observed when visitor traffic was high. During winter months when
visitor traffic decreased, rainfall and snowmelt runoff primarily caused
mobilization and removal of loose sediments covering bedrock surfaces.
Bedrock thin sections from each site were studied to assess the
influence of cement on erosion rates. Lower cement concentrations were
observed at the outcrop site with the greatest measured surface change.
Changes detected with SfM analyses demonstrate that human interactions
can influence erosion processes in a short time. While both natural and
human caused changes occurred on bedrock surfaces, precipitation created
greater measurable differences.