NEWRY – Damage from the July 11 catastrophic flooding that buried Bethel’s municipal water supply under tons of debris and destroyed Chapman Brook Watershed astounded a state geologist checking the area Wednesday for multiple landslides.
“In all my years of mapping Western Maine, I’ve never seen where a flood scoured a stream right down to hardpan glacial till,” Maine Geological Survey geologist Woodrow B. Thompson said while standing at about 1,500 feet above sea level in the watershed’s steep valley on the southwest side of Barker Mountain in Newry.
Evidence here indicated that the water was 20 to 25 feet deep at the height of flooding. It was much deeper farther downstream.
Thompson was looking at what geologists also call basal, or hard-as-nails, fine-grained sediment resulting from the melting of debris-rich ice in the bottom part of a glacier. He estimated that the last glacier left Maine about 14,000 to 15,000 years ago.
“This stuff is probably debris carried within the ice. As the ice melted, it settled out in layers. Normally, basal till is not so stony, because the stuff got ground up under the glacier into finer stuff. Usually, it’s really, really dense and hard and, a lot of times, the stones get ground up and scratched by the glacial abrasions,” he said.
In some places, along Chapman Brook, the brownish basal till was completely washed away, leaving either gray glacial-lake clay or bedrock.
“We know there was a glacial lake in Bethel at an elevation of about 700 to 800 feet above present day sea level. The clay here is typical of what settles out into a glacial lake. This (damage) is reminiscent of the flooding they had in Osgood, N.H. It’s amazing how localized this storm was. It’s spectacular! You don’t see this kind of thing very often,” Thompson said.
Streams today, like Chapman Brook, are eroding down through older sedimentary deposits of rivers and streams and possibly reactivating much older landslides that left behind alluvial fans – fan-shaped deposits of sediment formed at the point where a stream enters a valley, plain or another larger stream, he said.
“It’s possible during a major flood of every 100 or 500 years that these alluvial fans might get reactivated,” he added.
In the July 11 storm, which dumped between 6 to 8 inches of rain in 45 minutes on the northwest side of the main stem of Chapman Brook, Thompson only found evidence higher in the watershed of a basic flood.
“So far, I see nothing to call this an avalanche. It was just an hellacious flow and bank failures,” he said.
The farther downstream he walked, however, the more prevalent landslides of increasing size became. Bark high on trees was smashed off, and large trees were uprooted or undercut by flooding and tossed around like toothpicks. Several layered large clay and basal till banks were also exposed.
“It’s very impressive damage. It’s mixing with a lot of landslide debris farther downstream. It looks like flooding created several temporary impoundments behind downed trees and gravel piled up behind. Then, it would let go and flush, and that flush would be more powerful than the flood. It’s kind of like what happens when you’re busting a beaver dam,” Thompson said.
In several places, debris-flow landslides were caused when flooding undercut steep stream banks, then, the whole thing let go with an avalanche.
Slumping was another type of prevalent slide. Thompson said this indicates the slope was unstable for a long time, then, along comes a flood that takes out the toe of the slope, rotating one or more large blocks of clay downward.
“If you have an unstable slope or a landslide about to happen and remove the toe, that’s setting it up for an even more unstable situation where the slope will fail and you’ll get a landslide.
“This same kind of erosion must have been going on after the glaciers left. Today, when something catastrophic like this happens, by examining it, we get a better understanding of what happens and, two, we can educate the public about landslide erosion, so they don’t end up building in a vulnerable place,” Thompson said.
That’s why the Maine Geological Survey is currently seeking help from the public to identify landslides from 1987 to present.
But the flooding that buried the Bethel Water District’s Chapman Brook Reservoir, which was built in 1893, was an unpreventable freak of nature.
“The fact that the reservoir has been there for more than 100 years indicates that these things don’t come along often. The flood opened a can of worms, so, those destabilized banks have to re-vegetate. Hopefully, it will stabilize and get back to normal,” Thompson said.
Thanks to a federally-funded pilot study, the Maine Geological Survey is currently conducting a landslide inventory, studying and mapping slides in southern Maine from 1987 to present. It is seeking help from the public due to few reliable records of landslides, small ones of which, are common in Maine.
According to the Maine Geological Survey Landslide Inventory Questionnaire, the data will be used to update Maine’s inventory of landslides, which then can be used by state, county and local officials, consultants, and utilities to assist them in planning, regional landslide analysis, and hazard mitigation planning.
The Maine Geological Survey wants accurate information and locations of landslides of any type or size.
To help, visit http://www.maine.gov/doc/nrimc/mgs/slide-form.htm or call 287-2801 or e-mail [email protected]