What were chevaux-de-frise?
Credit: Independence Seaport Museum, https://explorepahistory.com/displayimage.php?imgId=1-2-209F
Chevaux-de-frise or, as the American forces at the time apparently referred to them, stackadoes, were obstructions of river passage that were designed to puncture the bottoms of the wooden ships of the time. They were constructed of long poles, tipped with iron, anchored in boxes weighted with rocks. The Pennsylvania Navy planted two fields of these obstructions, opposite Fort Billingsport and opposite Fort Mifflin, in the Delaware River downstream from Philadelphia.
The obstructions could be removed, with difficulty, but the ships doing the removal would be targets vulnerable to cannon ashore. Preventing the removal of these obstacles was a large part of the reason for being of the two forts. Above is a representation of a chevaux-de-frise in place in the river. This is a picture of a remanant of one of these obstacles from the museum at Fort Mifflin:
For the story of the siege of Fort Mifflin, which tried to prevent the British from clearing the river of the chevaux-de-frise, see Fort Mifflin Timeline.
For a fascinating account of a relatively recent discovery of one of these obstructions, see http://twipa.blogspot.com/2013/06/preservation-of-cheval-de-frise-from.html