Cotton Brook Landslide

A large landslide occurred on May 30 or 31, 2019 in the drainage area of Cotton Brook in the Mt. Mansfield State Forest in Waterbury. There are several popular trails in the area used for hiking, biking, and cross-country skiing. The Foster Trail was seriously impacted and is not usable in the vicinity of Cotton Brook. The main active landslide encompassed 12 acres with an additional 2.2. acres of detached blocks. In the days following the initial slide, the landslide area increased by more than ten percent. There is a high potential for additional failure making this a dangerous site.

The landslide resulted in massive sedimentation in Cotton Brook from the landslide location to the mouth of the brook on the west shore of Waterbury Reservoir. The new delta created in the Reservoir is approximately 2.8 acres in area. The Little River carried silt from the slide into the Winooski River.

Geologists visited the site one year later and took aerial photos using drones. The 2020 visit showed that the landslide continues to expand vertically and horizontally. The hypothesis of failure posed is that the landslide may have formed on glacial lake bottom deposits of very fine sand, silt, and silty clays, which had previously failed in an old landslide in Glacial Lake Winooski.

To view more drone photos, visit: https://dec.vermont.gov/geological-survey/hazards/landslides/cotton-brook.