Wetlands

Wetlands in the Portland Shire are most commonly found on dune topography and on basalt covered terrain. On the coastal plains are dune ridges which lie parallel to the present coastline and represent successive shorelines stranded by the retreat of the sea in Quaternary times. The oldest dunes consist of consolidated calcareous materials but some still carry unconsolidated siliceous sand. In paces, this has been blown inland to form sand sheets.

Wetlands are found in swales between both the siliceous and calcareous ridge systems. Some of the larger wetlands, such as Bridgewater Lakes, are remnants of prior lagoon systems, others are relict linear swamp alignments between dunes left by the ebbing of such lagoonal waters. Many semi-permanent wetlands are the result of underground water percolating through dunes and appearing where these dunes overlie impervious materials. Ephemeral wetlands occur in undulating dune topography where hollows fill temporarily after heavy rains. Occasionally wetlands my be found where ridges of lithified calcareous sand impede local drainage.

Wetlands which have developed on basalt topography are largely the result of disrupted or poorly developed drainage patterns. The Mount Clay area is covered by ash and siliceous sand, and in the Mount Richmond - Mount Kincaid - Mount Vandyke - Mount Sugarloaf districts, which are covered by extensive sheets of sand, wetlands are common since there is almost no external drainage.

Extensive swamp terrain has developed where pre-existing drainage systems have been disrupted or displaced by lava flows. South of Branxholme small residual swamps are remnants of a prior (Pliocene) drainage system disrupted by basalt flows (Boutakoff, 1963). The Condah, Homerton and Whittlebury Swamps are the result of the displacement of the Fitzroy River and headwater tributaries of Darlot Creek by the Tyrendarra Lava Flow which emanated from Mount Eccles. Lava from Mount Vandyke, Deception and the Eckersley group of volcanoes, combined with tectonic uplift, resulted in the displacement and eventual capture of the Old Crawford River by the Glenelg River. Three major river channels have been recognised, these are now marked by strings of ponds and swamps such as Dyes, Never Never and Wild Dog Swamps north-west of Heywood.

In the Portland district, circlar or elliptical wetlands termed sinkholes are found where the baslat topography is underlain by limestones. Solution of the limestone by water is thought to have resulted in the sagging and eventual collapse of the basalt into the limestone cavity below. Where these limestone cavities are also infilled by clays, clay blockage stops further water percolation from the surface, resulting in ponds or swamps.

Wetlands within the Portland Shire were considered to be of geomorphic significance if they displayed the action of geomorphological processes such as the effect of lava flows on pre-existing drainage patterns, the effect of the solution of underlying limestone topography, or the effect of calcareous or siliceous dune systems on drainage patterns. The physiography of some wetland sites provides evidence relating to the past sequence of geological/geomorphological events. Several may contain fossil pollen records within their sediments which could assist in the reconstruction of past climatic events. Other sites are considered to be representative of the landform units to be found within the Shire. Over 100 wetland sites were delineated on aerial photographs and most of these were briefly inspected in the field. Of these, 23 have been selected as wetlands of geomorphological significance.

As the alluvial soils of many wetland sites within the Shire constitute excellent agricultural soils when drained, many wetlands have been significantly altered. Drains have been dug to lower water tables and swamp vegetation cleared to provide pasture. Some of the larger areas such as Wright Swamp are now only recognisable as swamps by the alluvial aureoles which are still conspicuous on aerial photographs. Many smaller wetlands which only hold water after heavy rain have been cleared and ploughed. Some of the remaining smaller wetlands are grazed, particularly during dry summers, and the resulting trampling and desiccation reduces their value as wetland landform units.

To maintain the value of the wetland sites selected in this report, it is recommended that further drainage, reclamation, and the removal of alluvial material be prohibited. In addition, grazing on wetlands that sill retain extensive natural swamp vegetation should be phased out.