MOSENTHEIN ISLAND BLEW HOLE
Mosenthein Island
Mississippi River between Chouteau Island, Illinois and
North Riverfront Park, St. Louis, Missouri
N38.736306 W090.185728

ABOUT THIS WATERSHED
This Mosenthein Island cairn is constructed from rocks deposited by the Army Corps of Engineers as a “river training structure” and a barge rope that had floated downriver to the island. The boats and canoes that carried cleanup crews to the island did not have to compete with commercial traffic on this part of the river because all river traffic uses the Chain of Rocks Canal, built by the Army Corps of Engineers (opened 1953) to bypass the rocky ledges in the river bed. Boats enter the canal on the Illinois side of the river, north of the Highway 270 bridge, go through Lock #27, then reenter the main flow north of the McKinley Bridge.
The absence of big boats in this part of the Mississippi doesn’t mean that the river is domesticated; because of swift currents, only very experienced paddlers and boaters should attempt crossing from North Riverfront Park on the Missouri banks. The pools of clear water in the sand at the northern end of the island (called blowholes or blew holes) are evidence of the water’s pressure below the surface where underwater river currents slam into the island deep below the water’s surface. This rushing water creates and fills caves where the water pressure grows until it “blows” water vertically up to the surface, making these pools of clear, filtered water.
The river isn’t the only way to get to the island. In winter or low-water conditions, hikers can reach the island from the parking lot at the Old Chain of Rocks Bridge or from the Chouteau Island Nature Trail on the Illinois side. Mosenthein Island is the property of the Illinois Department of Natural Resources (IDNR), and hunting is permitted in season. Check with IDNR before you visit the island.
To learn about hunting on the island:

http://www.dnr.illinois.gov/hunting/FactSheets/Pages/HorseshoeLake-MadisonCORPS.aspx

For more information about the canal or blow holes: http://www.johnweeks.com/river_mississippi/pagesC/umissCM04.html

https://en.wikipedia.org/wiki/Blowhole_%28geology%29

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