Below water level the story is similar. Murray Cod need snags for spawning sites. Water quantity and quality dominate current concerns about the river. The willows are implicated in both. Because they are shallow-rooted, they take most, if not all, of their water from the topsoil or the river itself, and they appear to take a lot.
Willows add to salinity problems, too.
Their shallow roots don't reach down into the saline aquifers which supply the deeply rooted red gums. Willows skim only the upper, fresh water layers, allowing salt to rise and spill into the river. There are actually two kinds of willows. The weeping willow, Salix babylonica, is the tree made familiar by willow-pattern plates. It has a drooping aspect, whereas Salix fragilis is the more upright 'crack' willow.