Falmouth river’s restoration : Using Drones

The drone hovered at eye-level, rock steady, as if hung from invisible strings, its four propellers buzzing like a hive of angry wasps.

Pilot Erika Johnson ran through a mental checklist, her gloved fingers jabbing at an iPad mini, its screen shielded by a clip-on cowl from the glare of the sun. When she was satisfied, Johnson, a postdoctoral investigator in applied ocean physics and engineering at the Woods Hole Oceanographic Institution, sent the drone skyward with the flick of a finger to capture thermal images of the Coonamessett River bog system below.

Afterwards, a computer program would stitch the photos together into a seamless thermal map of the bogs that researchers hoped would reveal the location of groundwater springs where native sea brook trout could find vital refuge from summer heat and winter cold. The goal: gather information as part of a $3.5 million restoration of the river.

Revered by anglers for being larger and tastier than landlocked trout, sea brook trout moved into Cape rivers soon after the final glacier receded 10,000 to 12,000 years ago. They were popular with 19th century sport fishermen, and their demise had as much to do with overfishing as with changes to their habitat from damming rivers for cranberry cultivation and industry, resulting in warmer water and diminished water quality. Farmers also straightened the Coonamessett, clearing overhanging trees and brush and the deadwood that once gave the trout a place to hide from raptors.

Trees and brush also shaded the river, keeping areas cool enough for trout in summer.

” Right now, the only place cool enough is above Sandwich Road,” said Michael Scherer, a retired fisheries biologist who maintains a network of data loggers that record temperature along the river.

Scherer is a member of the Coonamessett River Trust, the group spearheading the restoration of the river after more than 300 years of use for farming and mills to grind corn and card wool.

Approximately 60 acres of the river valley will be part of the restoration, which includes the removal of two dams and the extensive widening of a culvert that passes under a roadway. The town bought the property for conservation in 1971. It was last farmed for cranberries in 2012.

With a $1.15 million grant from the National Oceanic and Atmospheric Administration, a diverse portfolio of smaller grants from state and private entities and a lot of donated labor, the trust is planning to break ground this summer, changing the course of the river and history.

” That’s where all the science and engineering and the experience come in,” said Nick Nelson, a fluvial geomorphologist and regional director for Inter-Fluve, a river and wetland restoration company serving as the primary consultant and designer for the Coonamessett project.

Video: Thermal imaging using Quadcopter

Excavators will scrape off 1 to 3 feet of sand, layered onto the topsoil by cranberry farmers to promote vine growth, with the hope that long-dormant seeds of cedars, red maple and other wetland-loving species are still viable after hundreds of years and will quickly repopulate the area once exposed. Contractors will reconstruct what scientists believe is the natural serpentine progress of the river given its water flow, the gentle slope of the land and the narrow valley. Scores of massive tree trunks are already stockpiled along the perimeter of the bogs, ready to be deployed in the sharp bends of the reconstructed river as deadwood that will help shade and protect fish, and create deeper, cooler pools as the current scours out sand around them.

The alterations will also benefit river herring, which migrate from the ocean into the ponds of the Coonamessett system to spawn before returning to the sea. The lack of vegetation overhead leaves them so exposed to predators that, unlike other Cape streams and rivers, they hide during the day and migrate at night. Last year, the Coonamessett had an estimated 75,000 river herring head upstream, one of the highest counts on the Cape.

The March 13 flight guided by Johnson was supposed to be hands-off, with the drone following a programmed flight path of plotted waypoints over the bogs. The extreme cold sapped the batteries and played havoc with data transfer of flight coordinates to the drone’s navigation system, so it was done the old-fashioned way, a manual flight using visual landmarks for navigation.

That flight did not yield images due to technical difficulties, but a subsequent flight Thursday captured over 1,500 images for processing.

The thermal images from the drone reveal where groundwater, possibly moving along ancient buried river or stream beds, is discharged into the Coonamessett. WHOI bought the drone and thermal camera for $17,000 to use in cooperative research, said Matthew Charette, an associate scientist in marine chemistry and geochemistry at WHOI and the incoming director of the WHOI Sea Grant program. Charette and Johnson were hired by the trust to help find the groundwater springs. The thermal imaging camera can distinguish the light wavelengths associated with a particular temperature and convert them into a photographic image, where objects are depicted in colored temperature gradients.

While surface waters in the Coonamessett can vary between 32 and 68 degrees, the Cape’s groundwater is a constant 55.4 degrees. When precipitation trickles downward from the surface layer, it is warmed or cooled as it passes through a thick band of unsaturated soil that maintains a constant temperature that is an average of the Cape’s annual temperature.

Drone technology has made thermal imaging accessible and relatively affordable. Back in 2000, it cost $30,000 for a plane to fly from Michigan and survey three ponds, Charette said. The thermal imaging study using the drone cost the trust around $3,000 he said, although some of that cost was offset by donated labor.

Brook trout need water that is cooler than 65 degrees to survive, and Scherer is hoping they will be able to either incorporate the seeps into the new design of the river or route them to chill the water in the river bends where the velocity is slower and more conducive to fish gathering to feed and spawn.

A restoration effort in the upper portion of the river by Steve Hurley, fisheries manager for the state Department of Fish and Wildlife, has been very successful, said Betsy Gladfelter of the Falmouth Conservation Commission.

“He believes, and we believe, that if we create cold water refuges in the lower part of the river, the trout will find them,” Gladfelter said.

That could put them close enough to Great Pond to migrate out into the saltwater estuary, where food is much more abundant, and allow them to grow to a size prized by anglers and better equipped to spawn.

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