The Keyword search is done on a word basis, ie. if your search for 'tree river' the search will be for either 'tree' or 'river', rather than the phrase 'tree river'.
For both tree and river use the format 'keyword1+keyword2', e.g. tree+river. Also note that additional plus signs will further refine the search, as in tree+river+eden.
Note that when searching in this manner it is necessary that there are no spaces between the keywords and the + sign.
To search for an exact match please use double quotes - eg "river eden". You may also combine such a search with another keyword, e.g. "river eden"+fish