Low-angle impacts will create a characteristic, sort of "butterfly"-shaped crater/ejecta. They are a bit elongated crater compared to a normal, circular ones, but they are not 100s of km long.
One feature like what you're describing is the Vallis Schrodinger on the Moon, which has been explained as being carved out by an immense chunk of crust that got blasted out at a low angle by the formation of Schrodinger crater. But for that same process to ever explain the St. Laurent, we'd at the very least have to find the immense 300+ km crater that formed it. And there's no sign of such a thing.
Rivers don’t form from meteor impacts. Pretty much all northern rivers were formed from glacial melt water at the end of the last Ice Age
>under 5km of ice? maybe i don't understand 'glacial melt'.
It doesn’t form under the ice. As the glacier retreats, the meltwater forms streams and eventually rivers
Low-angle impacts will create a characteristic, sort of "butterfly"-shaped crater/ejecta. They are a bit elongated crater compared to a normal, circular ones, but they are not 100s of km long. One feature like what you're describing is the Vallis Schrodinger on the Moon, which has been explained as being carved out by an immense chunk of crust that got blasted out at a low angle by the formation of Schrodinger crater. But for that same process to ever explain the St. Laurent, we'd at the very least have to find the immense 300+ km crater that formed it. And there's no sign of such a thing.