The Dorchester Mill Pond lies in a glacial spillway, which is a long, wide valley cut into the ground by meltwater from the retreating glacier at the end of the last Ice Age. The valley contained a creek which flowed northwards and joined the Thames River. The dam that was built at the location has changed the surface of the valley from a creek into a pond, in order to power a mill that once stood at this location. The mill is now long gone, but the dam and pond remain. The pond is gradually getting shallower over the years, however, due to silt.
Silt. Soft, gooey, mucky black pond bottom stuff. Decaying organic contents of silt smell like rotten eggs. Nature gives us silt, naturally. It is as normal as fallen leaves, grass clippings and fish poop.
Silt is defined as “loose sedimentary material with rock particles usually 1/20 millimeter or less in diameter.” The loosest of all soils, silt collects on top of more solid, clay based pond bottom soils. From there, water currents and wave action send silt into convenient areas for deposit. As the water in pond changes depths with the season, neighboring loose dirt can wash downward, eroding shorelines, and pitching fine particles into silt bars and banks, deposited near inflow areas.
As a pond ages, less dirt comes in, but more organic matter collects. Water composts leaves, twigs, clippings and fish waste, and plops these new deposits on top of older ones. As time goes, silt layers become deeper. Finally, we perceive silt to be a problem. How do we know? Ponds lose depth. Loose soils, silt, become home to cattails, reeds and rushes, usually in the upper parts of a pond…where water flows in. As silt mounts, cattails and reed families spread. Over time, a pond is covered with tall plants. With more years of age under your pond’s belt, soils collect and peek above the waterline, and terrestrial plants become dominant, The pond becomes land, again, as the shore moves inward.
Take a walk around the Mill Pond Trail, which is 3.3 km in length and will take 45 to 60 minutes to complete the circuit. Find 3 examples of "silting in", plants growing in silt, or structures contributing to the gradual filling in of the pond. What will eventually happen to the Mill Pond if this process continues?
Send the answers to your observations to norshley@gmail.com.
(Optional) Finally, take a photo of yourself or your GPSr unit somewhere along the Mill Pond Trail, with a scenic view in the background. You may post this photo along with your online log.