Blackwater is oxygen-depleted water caused by the decay of organic matter. The decay process darkens the water and the oxygen in the water is consumed, sometimes at a rate faster than it can be replenished. This can result in a low level of dissolved oxygen that may cause stress to fish, crayfish and other aquatic animals. When the dissolved oxygen reaches a very low level it can result in fish deaths (Murray Darling Basin Authority, n.d.).
Blackwater occurs naturally during flooding events in the Murray Darling Basin, and, despite it's harmful effect in the short term, can contribute to overall improvements in its ecosystems. This is because the breakdown of organic matter releases carbon and other nutrients into the system, boosting zooplankton and macroinvertebrate communities, in turn acting as food sources for fish, plants and other organisms.