Dams and weirs blocking migration of Australian bass both to estuaries and to the upper freshwater reaches of coastal rivers is the most potent cause of decline. Most coastal rivers now have dams and weirs on them. If Australian bass are prevented from migrating to estuaries for breeding by an impassable dam or weir, then they will die out above that dam or weir. Some dams or weirs exclude Australian bass from the vast majority of their habitat. It is estimated for example that Tallowa Dam on the Shoalhaven River, once an Australian bass stronghold, currently excludes wild Australian bass from more than 80% of their former habitat (in early 2010 however a "fish lift" was fitted to the dam). Dams and weirs also diminish or completely remove flood events required for effective breeding of adult bass and effective recruitment of juvenile Australian bass. A related issue is the myriad of other structures on coastal rivers such as poorly designed road crossings that (often needlessly) block migration of Australian bass.