Instead of forming a single swamp, these wetlands are made up of many swamps, none larger than 50 acres. They are spread across the woods, separated by porous ridges of sand and gravel laid down by the retreating glacier. The ridges allow water to seep down to the aquifer--the underground water-bearing beds of sand and gravel--which supplies the springs and wells of the area around the Great Woods. The swamps, releasing their water slowly, help maintain the water table of Mansfield, West Mansfield, Norton and Chartley.
One-third of the Great Woods has a water table at or near the surface of the ground, seven to nine months of the year. The U.S. Geological Survey has determined the water level in several private wells around the edge of the woods to be five to eighteen feet below ground, with an average of less than eleven feet.
In a year having average precipitation of 45 inches the 2300 acres of the Great Woods receive 2.8 billion gallons of rain and snow meltwater. The town of Mansfield in 1999 pumped an average of two million gallons of water per day. More than 30 times that amount falls on the Great Woods during a one-inch rainfall.
In Massachusetts, on the average, about half of the rainfall is absorbed into the ground, while the remaining half runs off. In the flat, swampy Great Woods, probably two-thirds of the rainfall, or nearly two billion gallons a year, percolates into the aquifer or is stored in swamps. Most of the remaining one third drains into the Rumford River on the northeast, Hodges Brook on the west or Great Brook in the center of the woods. Great Brook is notable because its entire course is situated in the woods until it empties into the Norton Reservoir, to which it contributes 19 percent of the inflow.
Oliver D. Diller, in Our Forest Resources, points out the important role that the forest plays during the winter. He explains that good forest cover influences snow accumulation and melt. Snow remains on the forest floor a few days to several weeks longer than on exposed areas nearby. As deep snows melt, much water percolates gradually into the forest soil where it finds its way to the water table. This process helps to reduce runoff and prevents silting in streams and reservoirs. Every winter the fact that the Great Woods is a forest, not open fields, house lots or paved commercial areas, helps raise the ground water table and increases the water supply available in Mansfield and Norton.
*Mr. Chase has camped, hunted, surveyed, mapped and observed plant and animal life in the Great Woods for over 60 years.