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1.
Jumping in the St. Clair River (between Sarnia and Port-Huron) from the breakwall at the point and letting the current take you under the BlueWater Bridges. This is typically done on the Canadian side of the border as they Americans are pussies and can't handle swimming in the middle of a frieght-ship route. Other activities surrounding the river-run include bridge jumps (from the base of the bridges anchors), and also Cove-runs (a river run which is twice the distance and starts at the Canatara Cove). Although typically viewed as dangerous, due to the current, possible undertow, waves, and large quantities of boats and ships that pass down the river, it is normal to view hundreds of people doing river-runs every day. They are also most popular at night between 9pm and 2am, just to add to the fun.
"Lets hit up the bridges for some river run tonight around midnight."

"Wow, the Americans are the biggest pussies in the world for not doing river runs. What type of retard can't handle this little swim."
by Sergant Dangles January 16, 2007
 
2.
(n). done on the eve of housing day by the bat-shit crazy freshmen of harvard; requires taking shots at the 12 houses the sorting hat may put blocking groups in. fires: optional. purpose: GETTING FUCKED UP
Jody: "heyy do you guys want to do river run and take the most shots at the houses we want to live in more for the next 3 years?"
by Gary Larry March 09, 2011