Question: Name the river whose mouth mountain is Southend-on-Sea and mouth place is Essex?
SPARQL:
SELECT DISTINCT ?uri WHERE {?uri <http://dbpedia.org/ontology/mouthMountain> <http://dbpedia.org/resource/Southend-on-Sea> . ?uri <http://dbpedia.org/ontology/mouthPlace> <http://dbpedia.org/resource/Essex>  . ?uri <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://dbpedia.org/ontology/River>}