 The crossing of the Saint Lawrence by the American troops was observed by Quebec militia , who alerted British troops at Trois @-@ Rivières . A local farmer led the Americans into a swamp , enabling the British to land additional forces in the village , and to establish positions behind the American army . After a brief exchange between an established British line and American troops emerging from the swamp , the Americans broke into a somewhat disorganized retreat . As some avenues of retreat were cut off , the British took a sizable number of prisoners , including General Thompson and much of his staff . 
