Biodiveristy_State of Environment report - Mission Beach Cassowaries

of the vital role they play in determining water quality. TheShire's wetlands used to be floodwater retention basins.This meant that they both slowed movement of and storedlarge volumes of water during excess rainfall, and helpedfilter sediments and nutrients from the water. Wetlandshave further important functions as fishery habitat/nurseryareas, and food source areas for fish.Since their destruction, runoff volumes have increased, andlower reaches of the river are subject to increasedsedimentation and increased flooding. The removal ofwetlands is an important part of the cause of sediment andnutrient problems affecting the Great Barrier Reef.Figure 2.1.3a - Changes to Wetlands in the JohnstoneRivers Catchment between 1951 and 1993Source: Russell and Hales (1993)Note: Scale depicted in the key does not apply hereIncreased sedimentation and flooding have impacts uponthe urban riparian areas in the Shire's towns, especiallyInnisfail, which is now looking at dredging its riverbeds.Historically, these river reaches were deep enough to allowfor large shipping. Now these reaches are so shallow theypose flood threats to urban areas.ConditionExtensive work has been conducted into the historicalrange of wetlands in the Shire (Russell and Hales 1993,Russell et al. 1996, Russell and Hales 1997). Work wasconducted for all four catchments in the Shire.They estimated the following changes to vegetation in theJohnstone Rivers catchment between 1951/2 and 1992(Table 2.1.3a). The areas were progressively cleared,drained and utilised for urban areas, agriculturalproduction, and grazing. The changes are also visuallyrepresented in Figure 2.1.3a.Table 2.1.3a - Changes to Wetland Vegetation Types inthe Johnstone Rivers CatchmentSource: Russell and Hales (1993)(Hectares) 1951 1992%changeMangroves 176 202 + 15%Melaleuca forests 1277 282 - 78%Mixed Melaleucacommunities462 258 - 44%Palm/Pandanus 439 160 - 64%Freshwater swamp/reeds 499 225 - 55%Total 2853 1127 - 60%The total area of wetlands in the Johnstone catchmentdecreased by approximately 60% over the 41 year period.All of the net losses were as the result of the reclamation offreshwater (non-mangrove) wetlands. The most significantlosses were of Melaleuca forest (78%) particularly to thesouth of the estuary, in the Ninds Creek Catchment. MixedMelaleuca communities also declined about 44%. Theother major freshwater wetland categories, palm/pandanusand freshwater swamp/reeds, were reduced by 64% and55% respectively. Freshwater wetlands to the north andwest of the river confluence have almost completelydisappeared. There was slight increase in the total area ofmangroves due mainly to colonisation on the southernforeshore and at the mouth of Ninds Creek. Drainage andreclamation works continue to threaten the remainingsection of the Ninds Creek wetland area.39