Where do the rivers from greater London flow into?
SELECT DISTINCT ?uri WHERE { ?x <http://dbpedia.org/property/sourceRegion> <http://dbpedia.org/resource/Greater_London> . ?x <http://dbpedia.org/property/mouthLocation> ?uri  . ?x <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://dbpedia.org/ontology/River>}
Greater_London