The Barmah Choke and the Narrows, a section of the Murray River where flow is restricted by a geological fault (the Cadell Fault), naturally cause the overflow of water into the Barmah Forest when the river flow is high.

Historically, the Barmah National Park and surrounding river red gum forests would flood naturally in winter and spring in most years, and river flows were very low in late-summer and autumn.
Since clearing for agriculture and the subsequent dam construction took place, the Murray River has undergone extensive flow regulation.
The construction of dams upstream from the Barmah National Park, from the 1920s onwards, has had a vast impact on the water flowing in the Murray River and instances of flooding, the flow of water is now highly regulated.

The Hume Dam was operational from 1936, the Yarrawonga Weir in 1939, and the Dartmouth Dam from 1979.

As a consequence of flow regulation, the winter and spring floods are now reduced, and of shorter duration than previously, with more low level flooding occurring in summer and autumn.
The increased incidences of smaller summer and autumn floods, which affect low-lying areas of Barmah National Park, are sometimes caused by heavy rains.
More often, they occur because there is sufficient rainfall for irrigated farmland between the Hume Dam and the Barmah Forest, consequently river diverters do not choose to take allocated water, Lake Mulwala (Yarrawonga Weir) also has inadequate storage, resulting in high level river flows referred to as "rainfall rejections".

These unseasonal high level river flows result in unseasonal flooding  in the Barmah-Millewa Forest, which has a significant effect on forest and wetland ecology, degrading wetlands by interfering with the natural drying-out phase and by disrupting nutrient cycling processes.
In recent years, unseasonal flooding is being controlled, and "environmental water" is being released to offset some of the detrimental effects of river regulation on the ecosystems along the lower reaches of the Murray River.
Management of environmental water supplied through the Murray-Darling basin to the Barmah–Millewa Forest is complex, and mainly under the control of the Murray-Darling Basin Authority.
In Victoria the Victorian Environmental Water Holder also manages environmental water allocations.

The supply of environmental water to Barmah Forest aims to:
Enhance the health of river red gum communities and aquatic vegetation in the wetlands and watercourses and on the floodplain, and promote the growth of floodplain marsh vegetation communities, particularly the extent of Moira grass
Maintain or increase available habitat for frogs and turtles
Provide feeding and nesting habitat for colonial nesting waterbirds
Provide native fish with access to a range of floodplain, riverine and refuge habitats including by delivering variable flows that promote spawning
Enable nutrient cycling (particularly carbon) between the floodplain and river through connectivity Provide early-season flushing of the lower floodplain to cycle nutrients during cooler conditions and reduce the risk of poor water quality events in summer
The Barmah Choke and the Narrows, a section of the Murray River where flow is restricted by a geological fault (the Cadell Fault), naturally cause the overflow of water into the Barmah Forest when the river flow is high.
Historically, the Barmah National Park and surrounding river red gum forests would flood naturally in winter and spring in most years, and river flows were very low in late-summer and autumn. Since clearing for agriculture and the subsequent dam construction took place, the Murray River has undergone extensive flow regulation. The construction of dams upstream from the Barmah National Park, from the 1920s onwards, has had a vast impact on the water flowing in the Murray River and instances of flooding, the flow of water is now highly regulated.
The Hume Dam was operational from 1936, the Yarrawonga Weir in 1939, and the Dartmouth Dam from 1979.
As a consequence of flow regulation, the winter and spring floods are now reduced, and of shorter duration than previously, with more low level flooding occurring in summer and autumn. The increased incidences of smaller summer and autumn floods, which affect low-lying areas of Barmah National Park, are sometimes caused by heavy rains. More often, they occur because there is sufficient rainfall for irrigated farmland between the Hume Dam and the Barmah Forest, consequently river diverters do not choose to take allocated water, Lake Mulwala (Yarrawonga Weir) also has inadequate storage, resulting in high level river flows referred to as "rainfall rejections".
These unseasonal high level river flows result in unseasonal flooding  in the Barmah-Millewa Forest, which has a significant effect on forest and wetland ecology, degrading wetlands by interfering with the natural drying-out phase and by disrupting nutrient cycling processes. In recent years, unseasonal flooding is being controlled, and "environmental water" is being released to offset some of the detrimental effects of river regulation on the ecosystems along the lower reaches of the Murray River. Management of environmental water supplied through the Murray-Darling basin to the Barmah–Millewa Forest is complex, and mainly under the control of the Murray-Darling Basin Authority. In Victoria the Victorian Environmental Water Holder also manages environmental water allocations.
The supply of environmental water to Barmah Forest aims to:
Enhance the health of river red gum communities and aquatic vegetation in the wetlands and watercourses and on the floodplain, and promote the growth of floodplain marsh vegetation communities, particularly the extent of Moira grass
Maintain or increase available habitat for frogs and turtles
Provide feeding and nesting habitat for colonial nesting waterbirds
Provide native fish with access to a range of floodplain, riverine and refuge habitats including by delivering variable flows that promote spawning
Enable nutrient cycling (particularly carbon) between the floodplain and river through connectivity Provide early-season flushing of the lower floodplain to cycle nutrients during cooler conditions and reduce the risk of poor water quality events in summer
Barmah National Park