Upwelling
The major upwellings in the ocean are associated with the divergence of currents that bring deeper, colder, nutrient rich waters to the surface. There are at least five types of upwelling: coastal upwelling, large-scale wind-driven upwelling in the ocean interior, upwelling associated with eddies, topographically-associated upwelling, and broad-diffusive upwelling in the ocean interior.
Murray cod
Blackwater events are often described as “natural” events—while there are some historical records of relatively severe events in smaller, more ephemeral systems (e.g. lower Lachlan, upper Darling), there is no record of severe events in the Murray River and its largest southern tributaries before water extraction and river regulation. In the Murray and large southern tributaries, very severe large-scale blackwater events are a relatively new but recurring phenomenon and appear to be an effect of river regulation curtailing the winter/spring flood events that formerly swept leaf litter away annually, exacerbated by long-term declines in rainfall and recurring prolonged drought events.