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