The sign -- "risk of drowning" -- outside one of Rio de Janeiro's freshwater reservoirs looks like a joke: there's no water here left to drown in.

Instead, the Saracuruna reservoir near Duque de Caxias, outside Rio, is an expanse of sand, mud and vegetation. Four stray dogs scamper and cattle come to drink from a stream still running through the middle.

"It's been a long time since there was any water here," said a security guard walking up the dry bed to order AFP journalists away on Friday.

The scene at Saracuruna is repeated across much of eastern Brazil between Rio and the megacity of Sao Paulo, with reservoirs and rivers running dry and authorities scrambling to avoid having to impose rationing.

Rio de Janeiro state's environmental department blames "the worst drought in 85 years" for the crisis, while independent activists say decades of bad policy is equally culpable.

Although the southern tropical rainy season is just beginning, scientists fear that the El Nino weather phenomenon active this year may disrupt that hoped for relief from the sky, leaving tens of millions of people at risk.

Daily water access, potential disruption of the 2016 Rio Olympics, crop irrigation, and the running of Brazil's hydroelectric industry, which provides for 75 percent of the country's power, are all at stake.

And with dire water shortages also breaking out as far afield as California and China, the crisis could also be a harbinger of wider trouble to come.

- 'We used to fish' -

In Xerem, a neighborhood of Duque de Caxias, about 31 miles(50 kilometers) from Rio, locals talk in disbelief about what remains of the river that until about three years ago rushed through to the Saracuruna reservoir, which was built principally to serve a Petrobras oil refinery.

"We used to fish there, take swims," said Renato Tomaz, 42. "There was a lot of water."

Only a shallow, almost motionless stream between two-meter deep banks thick with plants remains.

"It's not a river anymore. It's a forest," Tomaz said.

Scale that havoc up on a national level and you get the picture of what's happening along the huge system of reservoirs stretching through Sao Paulo and Rio states.

The Paraibuna reservoir, biggest in the chain serving Rio de Janeiro, is heading inexorably to dead pool status, meaning the remaining water is not usable.

In all, the four main reservoirs of the Paraiba do Sul network dipped this month to less than six percent of active volume.

The Cantareira system feeding Sao Paulo is in only slightly better shape, with 16 percent of active water left. A graphic on the water authority's website shows the dial hovering above the red danger zone.

And the southeast is not the only area suffering. The biggest reservoir of the northeast, Sobradinho, is reported to have fallen to less than six percent active capacity.

- Crisis? No, it's worse -

The idea of drought in Brazil might sound ridiculous.

Latin America's biggest country is one of the world's great sources for fresh water, accounting for about 12 percent of supplies.

However, much of that water is locked into the mighty Amazon river in the north, of little use to greater Sao Paulo's 20 million and Rio de Janeiro's approximately 10 million people.

And even without the current drought, environmental experts say, government mismanagement has been enough to bring Brazil to the brink of disaster.

"It's much more serious than a crisis. A crisis is something that ends eventually, but this is structural," said Rio-based environmentalist Sergio Ricardo.

"You have a prolonged lack of rainfall but also prolonged bad management that has left Rio completely vulnerable."

Those mistakes range from inefficient exploitation of water supplies to relentless pollution of the precious resource.

For example in greater Rio, 80 percent of sewage is dumped untreated, much of that into rivers, Ricardo said.

Mario Moscatelli, another prominent expert on water issues, said blaming the drought alone misses the huge man-made damage.

"Beyond problems linked to global climate change and regional effects like El Nino, the domestic causes for the degradation of our bodies of water continue," he said.

"Given the signs from nature, it's going to get much worse -- and the government response is always timid and always reactive rather than preventative."