From where does the river start, which flows into the Conowingo dam?
SELECT DISTINCT ?uri WHERE { ?x <http://dbpedia.org/ontology/riverMouth> <http://dbpedia.org/resource/Conowingo_Dam> . ?x <http://dbpedia.org/ontology/source> ?uri  . ?x <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://dbpedia.org/ontology/River>}
Conowingo_Dam