Answer:
The Northmen filled the Seine with so many boats that the river could not be seen.
Step-by-step explanation:
The passage describes how the Northmen arrived in Paris with a large fleet of 700 sailing ships, not counting the smaller boats called barques. The Seine River was so filled with these boats that it lined the river for more than two leagues (a league is a unit of distance, approximately equivalent to three miles). The passage emphasizes the overwhelming presence of the Northmen's boats in the Seine River, to the extent that the river itself could not be seen due to the sheer number of vessels. This conveys the central idea that the Northmen's fleet was so extensive that it filled and obscured the Seine River, creating a striking image of their overwhelming presence in the area.