The arithmetic of Memphremagog is wonderfully lopsided. Seventy-three percent of the lake's surface area lies in Quebec, but three-quarters of its watershed sits in Vermont. This means most of the water feeding the lake originates in one country while most of the lake itself belongs to another - a hydrological arrangement that has complicated environmental management for over a century. Four Vermont rivers empty directly into the lake: the Clyde, Barton, Black, and Johns. The Magog River drains it northeast into Quebec, where the Memphremagog Dam generates hydroelectric power and regulates water levels under a 1935 treaty between the United States and Canada. The lake bottom drops dramatically on the Canadian side, plunging opposite Mont Owl's Head to its maximum recorded depth near Jewett Point, making it the third deepest lake in Vermont despite being mostly in Quebec.

For a freshwater lake in the rural borderlands, Memphremagog once boasted a surprising maritime infrastructure. At least three lighthouses stood on the Vermont shore and seven on the Quebec side, most of the Canadian ones established in 1878. The Maxfield Point Light was a conical cast-iron tower 25 feet tall, its fixed white light visible for ten miles. The Newport Wharf Light was a red steel skeleton on a concrete foundation. Whipple Point Light, a hexagonal wooden tower on a pier, was deactivated around 1906. On the Quebec side, the Witch Shoal Lighthouse has been destroyed three times - by ice in 1933, swept away by spring ice again in 1978, and since 1980 replaced by a seasonal mast installed only from May to October. These lights guided the paddlewheelers that once plied the lake: the Mountain Maid from 1850 to 1870, the Lady of the Lake from 1867 to 1917, and the Anthemis, built in 1909 to carry 300 passengers.

The lake's history runs deeper than resort tourism. In 1753, Abenaki brought the ransomed John Stark - who would later become a general in the American Revolution and coin New Hampshire's state motto, 'Live Free or Die' - down the lake to the site where Newport now stands. Six years later, Rogers' Rangers retreated south along the lake after their infamous raid on the Abenaki village of Saint-Francis in 1759, splitting up along the east shore to evade pursuing forces. The first accurate map depicting the lake appeared in 1776, marking the beginning of settler interest in a body of water the Abenaki had known intimately for centuries. The name Memphremagog itself comes from the Algonkian Memrahabegek, meaning 'where there is a big expanse of water' - a description that remains as accurate as the day it was first spoken.

Every proper lake needs a monster, and Memphremagog has Memphre. Sightings of the cryptid date to the 18th century and continue through modern times - a report surfaced as recently as 2000. The creature joins a crowded field of North American lake monsters, from Champlain's Champ to Okanagan's Ogopogo, but Memphre has the advantage of inhabiting a genuinely deep and murky lake that straddles an international border, making comprehensive sonar surveys a diplomatic as well as scientific challenge. Whether Memphre is a large fish, a misidentified log, or something stranger, the folklore has embedded itself into the cultural identity of both the Vermont and Quebec communities along the shore. The lake has also inspired fiction: Chris Bohjalian set a nuclear meltdown here, Kenneth Roberts wrote Rogers' Rangers retreating along its banks, and the 1986 film The Decline of the American Empire was shot on its shores.

The lake that provides drinking water for 200,000 people faces a persistent tension between its ecological importance and the agricultural economy of its watershed. A 2010 study found that while farms account for just 16 percent of total land use in the drainage area, they produce 44 percent of the nutrient runoff. Phosphorus levels measured at 17 micrograms per liter - 18 percent above baseline standards for a healthy lake. The Lake Memphremagog Watershed Association, formed in 1994, has worked to address these issues, but progress is complicated by the cross-border nature of the problem. Vermont invested $10 million in clean water projects from 2016 to 2021, yet the lake still fails to meet water quality standards. In September 2020, a sewage treatment facility in Newport leaked roughly 7,000 gallons of effluent directly into the lake - a reminder that the Abenaki's 'big expanse of water' remains both precious and vulnerable.