As water tables dissolve limestone, caverns are created and the base level drops. Most caverns are formed by dissolution of the limestone in bedrock. Caverns are in karst or rocky regions and do not always contain entrances.
(Cavern in Suva, Fiji formed by dissolution taking away the limestone.)
Rivers are fluvial processes that cause erosion or deposition on the river beds. Rivers that look muddy are suspended loads which include sediment that is carried in the body of the flow. Since there are pieces of sediment in the water, it is not as clear as dissolved loads.
(Muddy suspended load river going from Nadi to Suva, Fiji)
Fiji has many littoral zones which is from the high water mark to the shore, which is subbmerged in water. In a few areas you can also see tombolos which are depositional landforms that extend from the shore out into the water.
(On this Fiji littoral zone you can see a tombolo which just looks like an extended beach out into the water.)
http://www.riversfiji.com/
http://gsabulletin.gsapubs.org/content/119/3-4/476.abstract
http://www.bride.ca/wedding-ideas/index.cfm/2009/7/22/Winter-Honeymoon-Destinations-Fiji
http://www.wiesiu.com/fiji_photos_gene.htm
http://www.fiji.beach-pacific.com/pictures.html