Grand Lake St. Marys

At a glance

Before the lake

Grand Lake St. Marys was built to solve a summit-level water problem: the Miami and Erie Canal, connecting the Ohio River at Cincinnati to Lake Erie at Toledo, needed a steady water supply across the flat, poorly watered divide between the Great Miami and Maumee/St. Marys watersheds in west-central Ohio. The state dammed the headwaters area between the Wabash and St. Marys rivers, flooding a low, swampy prairie between them.

Building the dam

About 1,700 men — consistently described across sources as mostly Irish and German immigrants — built the east and west embankments entirely by hand: picks, shovels, wheelbarrows, no machinery, for roughly 30 cents a day plus, according to local tradition, "a jigger of whiskey" meant to ward off the malaria endemic to the construction camps. A Wright State University archival project has specifically corrected the popular image of workers "digging" the lake out of the swamp: citing an 1981 EPA report and Henry Howe's 19th-century History of Ohio, the lake was instead impounded behind two raised earthen embankments — one on Beaver Creek at the west end, one on Chickasaw Creek at the east — flooding the low ground behind them rather than excavating a basin.

At completion the reservoir covered roughly 17,000-17,500 acres, about 9 miles long by 3 miles wide with 52 miles of shoreline — the largest artificial lake in the world at the time, a title it held until later reservoirs surpassed it. Construction cost is most often cited at $600,000, though an EPA report and a state historical marker independently converge on a more precise $528,000-$529,000 figure for completion of the east bulkhead in 1845. No primary tract-by-tract land-acquisition record — naming individual owners or compensation amounts — has been located for the original 1837-1845 taking, and no drowned town or settlement within the footprint has been identified; the flooded land is consistently described as low prairie and swamp rather than farmland or a settled townsite.

That taking was contentious enough to provoke violence. Within a few years of the lake filling, west-side residents living next to the new, stagnant, mosquito-breeding wetland complained of the stench and of illness, and some had still not been paid for land that now lay under water. They petitioned the state's Board of Public Works in Piqua and were told, in effect, to "help yourselves if you can." When word came that residents meant to cut the bank themselves, the Board threatened that the Piqua guard would "be with you and rout you on that day." Four days later, they did it anyway: a group cut a channel through the west embankment, draining the lake into the land it had flooded. The cut caused more than $17,000 in repair costs and 34 arrests — sweeping up the county sheriff, judges, the clerk, auditor, treasurer, recorder and surveyor alongside farmers and merchants. The grand jury declined to indict anyone. Sixty-one years later, on August 22, 1904, someone dynamited the east bulkhead gates near St. Marys — felt nine miles away in Celina — apparently for the same underlying reason: a wish to drain the lake and reclaim the farmland beneath it. No one was ever identified for either act.

Stories

Annie Oakley at the Celina Chautauqua

The Celina Chautauqua resort, near today's Mercelina Park, billed itself as "the finest pleasure resort in Ohio," hosting entertainers, dramatists, theologians and choral groups. Buffalo Bill's Wild West Show performed there, with Annie Oakley among its performers — meaning one of the most famous sharpshooters in American history likely stood on the shore of a lake that local farmers had twice tried to drain by force.

The first American offshore oil wells

Regional oil discovery followed the Lima-Indiana Trend, found near Findlay in 1884. The first well at Grand Lake was drilled in Celina in June 1886; a month later the first went in on the St. Marys side, and locals began drilling in their own backyards along the beach. Around 1891, operators extended drilling out over the lake itself, building derricks on wooden cribs and platforms and drilling from floating rigs — among the earliest freshwater offshore oil drilling in the United States, predating the commonly cited "first offshore drilling" claim for Caddo Lake, Louisiana in 1911 by roughly two decades. By 1915, more than 150 oil and gas wells had been drilled in and around the lake, some producing 25-250 barrels a day.

Devil's Backbone and the death of Gordon State Park

Gordon State Park, a privately run amusement park on 13 acres of the east shore, opened in 1924. Its signature attraction was Devil's Backbone, a wooden roller coaster about 90 feet high and 2,500 feet long, said to be the second-largest in Ohio at the time; a 300-by-50-foot dance pavilion drew 45,000 visitors over the Fourth of July weekend in 1926. Two fires, a tornado, and the Great Depression closed the park in 1933, within a decade of its peak.

The Hoedag

Local lore recalls a lake monster called the Hoedag, first reported in 1912 — roughly three-quarters the size of an elephant, with a serpentine body, chicken-like feet and mismatched-colored eyes. It dates from squarely the middle of the lake's oil-boom and early resort eras, when the shoreline was full of sportsmen's camps and tall tales.

A lake that keeps changing its name

The lake has carried several names: Lake Mercer (still used by the 1920 Encyclopaedia Britannica), Mercer Reservoir, Grand Reservoir, Lake Celina and Lake St. Marys, before "Grand Lake St. Marys" became the everyday compromise that never won full official sanction — the U.S. Geological Survey's Board on Geographic Names still lists the lake simply as "Grand Lake." Local newspapers reportedly threatened legal action against each other over the naming question, with roots reaching back to the 1820s-40s, when St. Marys lost county-seat status.

The lake since

A 1913 flood, part of the catastrophic statewide Great Flood, overwhelmed the Miami and Erie Canal system and much of St. Marys and Wapakoneta; the canal, already losing ground to railroads, never fully recovered, and a new spillway was built at Beaver Creek in 1914. In 1997, the state widened that western spillway from 39 to 500 feet. Downstream landowners began experiencing worse, more frequent flooding, and sued the Ohio Department of Natural Resources. In State ex rel. Doner v. Zody (December 1, 2011), the Ohio Supreme Court ruled the widening was a compensable taking and ordered ODNR to begin formal appropriation proceedings to pay the affected owners — the Ohio Farm Bureau had filed in their support. The two dam embankments themselves have separately drawn dam-safety attention: a 2019 Ohio Controlling Board action approved $584,186 for an interim risk-reduction rehabilitation of the west embankment and an engineering assessment of the east spillway, though both structures still carried a "Poor" condition rating as of a 2025 inspection.

The lake's defining modern crisis was water quality. National monitoring ranked its watershed among the third-highest in the nation for microcystin by 2009; the summer of 2010 brought a severe cyanobacteria bloom that killed three dogs and sickened at least 21 people, including one case of temporary blindness, driving away tourists at the height of the season. On January 18, 2011, ODNR designated the watershed "in distress," triggering mandatory manure-management rules: nutrient-management plans for large operations, a winter manure-application ban from December 15 to March 1, and 120 days of required manure storage. A 2018 peer-reviewed study found the rules cut nutrient loads 5-35% overall, and 20-60% during the December-March window specifically. The state also ran large-scale alum treatments in 2011 and 2012 covering roughly 30-40% of the lake's area, at a cost of $3-5 million; results were mixed, with a later study finding the phosphorus-binding treatment ineffective in a semi-enclosed part of the lake. Recovery has instead come mainly from restored wetlands: Prairie Creek's wetlands, tracked 2017-2024, processed 4.44 million cubic meters of water with 56% total-phosphorus and 81% soluble-reactive-phosphorus reduction, and by 2025 all four state-park beaches were reading below 0.4 micrograms per liter of microcystin with no active advisories — a dramatic turnaround after a decade and a half of blooms.

The water

The City of Celina (population about 11,000) draws its entire municipal drinking-water supply from Grand Lake St. Marys, with no groundwater backup. After an exhaustive multiyear study of alternatives, the city decided in 2018 to keep the lake as its source rather than switch; its treatment plant has since added ozone treatment (1990s, removing a musty taste-and-odor problem), UV disinfection (mid-2010s, protecting against cyanotoxin byproducts), and more recently a dissolved air flotation system for removing algal cells before treatment. St. Marys, the city at the lake's east end, does not appear to draw its own supply directly from the lake.

Ohio EPA rates the lake itself, and every tributary in its immediate watershed, "Not Supporting" for nutrients and pathogens; every one of the lake's four designated uses — drinking water, aquatic life, recreation and fish consumption — is listed "Not Supporting." Treatment wetlands built on four main tributaries (Coldwater Creek, Prairie Creek, Beaver Creek and Big Chickasaw Creek) processed more than 760 million gallons of water in 2025 alone. The lake sits almost exactly on the continental divide between the Wabash River (Mississippi/Ohio River basin) and the Maumee/St. Marys River (Lake Erie basin) — the reason it needed two separate embankments at opposite ends, one controlling each side of the divide.

Sources

• Lake Improvement Association, full history
• St. Marys Develops, life in St. Marys history
• City of St. Marys, history
• Grand Lake St. Marys State Park, Wikipedia
• Two Sides to Every Story: naming rivalry, Mercer County Outlook
• The Grand Lake Oil Boom, Daily Standard
• Ohio Offshore Wells, American Oil & Gas Historical Society
• State ex rel. Doner v. Zody, Ohio Supreme Court opinion, 2011-Ohio-6117
• Troubled Water, Dayton Daily News investigative series
• Grand Lake Saint Marys watershed program, Ohio Department of Agriculture
• Lower microcystin toxin levels signal progress, hometownstations.com
• The Lake is Alright: bounces back by going back, WYSO
• Grand Lake St. Marys Archive and History, Wright State Lake Campus
• Ohio Ag Net, Controlling Board dam-rehabilitation funding, 2019