OARS River Log | By Zoe Green, OARS 2024 Ecological Restoration Volunteer | Published July 25, 2024

From Westborough to Newburyport

The Sudbury and Assabet rivers both originate in Westborough, Massachusetts, flowing northward until they converge at Egg Rock in Concord, where they join to form the Concord River. From this junction, the Concord River winds its way to Lowell, merging with the Merrimack River and eventually reaching the Atlantic Ocean at Plum Island in Newburyport. This region, known as the Concord Watershed or Concord Basin, owes its current form to historical glacial activity. How can we infer the rivers’ history simply by observing them? Let me explain.

Egg Rock: Why a Rock Formed Beneath the Earth’s Surface Is Now Plain Sight

Egg Rock is a famous landmark of the Concord River. It boasts a rounded shape and inscriptions commemorating Concord’s history. However, this rock’s history goes back much further in time than the 1885 engravings. Egg Rock is a type of rock called Straw Hollow diorite, an intrusive igneous rock formed when magma cools and solidifies beneath Earth’s crust. You can tell it is an intrusive igneous rock by looking at it up close; it has a spotted appearance (gray with white crystals) and a coarse-grained texture.

Intrusive igneous rocks reach the surface in two ways: either erosion and uplift (physical and chemical) weathering or tectonic plate movement. During the Silurian period, around 443.8 million years ago, tectonic activity caused the microcontinent of Avalonia to move towards the North American plate of Laurentia (proto-North America). Over time, as these two plates continued to move towards each other, the heat and pressure from the eventual tectonic collision contributed to the formation of igneous rocks. Further pressure from the two plates converging caused some of the rocks under Earth’s crust to come to the surface and the crust to buckle and fold, leading to the uplift of these rock formations. This is how rocks formed deep within the Earth, such as Egg Rock, got closer to the surface, where erosion slowly removed overlying materials to further expose the uplifted rock we can observe today.

Glacial History: The Natural Forces that Shaped the Rivers’ Current Characteristics

About 10,000 to 15,000 years ago, during the Pleistocene epoch, the Laurentide Ice Sheet covered much of Canada and the northern United States. As it advanced southward, the massive ice sheet exerted tremendous pressure on the bedrock beneath, causing significant erosion and breaking the bedrock into smaller sediments. Acting like a colossal bulldozer, the ice scraped away soil, rocks, and other materials, depositing these sediments along its path. The combined processes of erosion and deposition, influenced by the underlying topography, shaped the region’s drainage patterns. As the glacier retreated, it left behind channels carved by meltwater and depressions that eventually evolved into river valleys and basins. The natural flow paths created by these glacial deposits determined the courses of rivers such as the Sudbury, Assabet, and Concord.

The Older the River, the More Pronounced the Meanders

The Sudbury, Assabet, and Concord rivers are relatively old rivers with a rich geological history. Today, the Concord River Basin in Massachusetts showcases characteristics indicative of its glacial past. Prominent landmarks include shallow bedrock, kettle ponds, drumlins (low, oval hills), kames (irregularly shaped hills), and thick stratified drift deposits. As the massive ice sheet that once covered much of North America melted away, the Sudbury, Assabet, and Concord rivers emerged, carving their paths through a landscape shaped by millennia of glacial activity.

These rivers’ large floodplains, low gradients, and slow-moving waters are all signs of an older river. A river’s dramatic meandering, a result of continuous erosion and deposition, showcases its dynamic and evolving nature. The outsides of meanders (cutbanks) experience erosion, while the insides (point bars) grow due to sediment deposition, showcasing the river’ dynamic and evolving nature.

How the Present Landscape Affects the Rivers’ Discharge (Flow)

Discharge refers to the amount of water flowing through a river or stream over a specific time, usually measured in cubic feet per second (cfs) or cubic meters per second (cms). It’s essentially the river’s “flow rate,” indicating how much water passes through a certain point in the river at a given moment. This is calculated by multiplying the rivers’ cross-sectional area by the water’s velocity. Think of it as the river’s “flow rate” or “Discharge of flow”—how quickly the water moves downstream and how much it is flowing. A high discharge means a large volume of water moving quickly, while a low discharge indicates less water moving more slowly.

The landscape—including the slope, shape, and geology of the land—influences how water moves through the watershed and affects the discharge of the rivers. Steeper slopes and impermeable surfaces lead to faster runoff and higher discharge. The drainage area, where surface water and precipitation collect, also plays a crucial role. The Concord River has a drainage area of 400 square miles, which is much larger than that of the Assabet (116 square miles) and the Sudbury (106 square miles). This larger drainage area results in a higher volume of water and, consequently, a higher annual discharge of 883.5 cfs.

The Sudbury and Assabet rivers flow from the southern to the northern part of the basin and originate at higher elevations, naturally descending toward the Concord River as they follow the pull of gravity. The Assabet River, originating at a higher elevation than the Sudbury, flows faster and has a higher discharge of 280.2 cfs despite its smaller drainage area. In comparison, the Sudbury River has a discharge of 251.6 cfs. These are just annual averages, so it’s worth remembering that actual daily measurements change depending on weather patterns and precipitation. Discharge measurements are measured at gauging stations, which are sites on a river where hydrological data is observed, but these measurements differ depending on where you are on the river. While elevation changes and channel size are natural factors that affect flow over time, man-made features like dams can drastically alter geological and hydrological features quickly.

The glacial past carved out the river landscape that we see today, a landscape that will continue to be shaped by erosion and deposition for eons to come. Beyond the already-complex narrative of human interaction chiseled onto Egg Rock, there is a deeper history embedded in this river’s geology, a story that will reveal itself to all who know how to read the landscape!