Which river starts from the county seat of Heathsville, Virginia?
SELECT DISTINCT ?uri WHERE { ?x <http://dbpedia.org/ontology/countySeat> <http://dbpedia.org/resource/Heathsville,_Virginia> . ?uri <http://dbpedia.org/property/mouthLocation> ?x  . ?uri <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://dbpedia.org/ontology/River>}
Heathsville,_Virginia