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