What is the river that falls into North Sea and Thames Estuary?
SELECT DISTINCT ?uri WHERE { ?uri <http://dbpedia.org/ontology/riverMouth> <http://dbpedia.org/resource/North_Sea> . ?uri <http://dbpedia.org/ontology/riverMouth> <http://dbpedia.org/resource/Thames_Estuary> . ?uri <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://dbpedia.org/ontology/River>}
North_Sea Thames_Estuary