What is the river with mouth in Southend-on-Sea?
SELECT DISTINCT ?uri WHERE {?uri <http://dbpedia.org/ontology/mouthMountain> <http://dbpedia.org/resource/Southend-on-Sea>  . ?uri <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://dbpedia.org/ontology/River>}
Southend-on-Sea