The Gregory River located in the Mackay/ Whitsundays region is one of Queensland’s best kept secrets. Its crystal clear waters and lush riparian rainforest are home to an abundance of unique and threatened species including the endangered Proserpine rock wallaby and the highly enigmatic jungle perch. Its headwaters begin in the rainforest clad mountains of Dryander National Park before meandering through grazing and sugar cane farms and flowing into the declared Fish Habitat Protection Area of Edgecumbe Bay and eventually the World Heritage Great Barrier Reef Marine Park. The protection afforded by National Parks and Fish Habitat Areas combined with environmentally sustainable farming practices has resulted in one of the most intact and connected river ecosystems in the Mackay/Whitsundays region. Its pristine freshwaters and diverse in-stream habitats (Figure 1) provide ideal nursery environments for many important recreational and commercial fish species, including; barramundi, mangrove jack and sea mullet.
But even great rivers far away from highly populated areas have felt the brunt of human induced change, in this case a small 1 m high gauging weir in the upper reaches of the catchment (Figure 2). The weir was constructed to monitor river flows and enable the sustainable use of water resources but it also formed a barrier to fish migration. To help facilitate fish passage past the barrier a fishway was constructed at the base of the weir. Unfortunately, after the construction of the fishway the Gregory River experienced consecutive large flood events that scoured away the downstream substrate, causing the pool beneath the fishway to recede by half a metre. The lowering of the downstream pool meant that during average flow conditions fish were unable to enter the fishway and migrate upstream. The large flood events also caused previously cleared downstream sections of river bank to erode and slump (Figure 3), leaving excess sediment to wash down the catchment.
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