Coastal features of the Warren River outlet in the region of the Yeagarup Dunes along the wave- and wind-dominated sandy southern coast of Western Australia. Evident in this image, from left to right, is (1) the steep front of the dune terrain of themainland dune coast with the steep sandy slope formed by wave erosion during storms, and by the prevailing strong winds; this dune front forms the main shore along this sector of sandy coast; (2) a shore-parallel lagoon that is a river channel cut into the upper part of the beach, now impounded by a beach berm; the waters of the shoreparallel lagoon are fresh to weakly brackish and tannin-stained having been recently derived from river flooding; when his image was taken, the lagoon was two metres deep, its width some 75-100 m, and it was several hundred metres long - the lagoon was detached from the main more permanent river outlet; (3) to the far right is the beach berm that is barring the lagoon; seaward of the beach berm is seawater of the Southern Ocean with normal marine salinity.

This type of shore-parallel lagoon of berm-impounded river water results from the interplay of floodwater discharge of tannin-stained freshwater from the Warren River that meets a high-energy wave-dominated coast. Normally, the river would discharge its flood waters directly into the ocean but, under prevailing wave conditions, a bar-and-berm is continually being constructed at times sufficiently to bar the river discharge; river flow then is diverted laterally (usually westwards) for hundreds of metres and up to 2500 m along a depression leeward of a shore-parallel berm system, carving a channel until it reaches a location where it can cut across the berm and form a new outlet. With concomitant decreasing river flow (as hinterland flooding recedes) and the prevailing building of a beach berm, the new river outlet is sealed, and the recently carved linear channel with its riverine water is trapped between the main more permanentmouth and its recently formed diverted mouth to form the tannin-stained freshwater body. With ongoing building of the beach berm and aeolian transport, the connection between the lagoon and the more permanent river channel is lost and the lagoon becomes an isolated depression until beach processes and aeolian transport fill and bury it (Semeniuk and Semeniuk, 2011). (Photograph taken November 2009 by Vic Semeniuk, V & C Semeniuk Research Group, Warwick, Western Australia.)