As the river between Wodonga and Echuca was elevated to ''red alert'' for algal bloom, there were concerns the risks to human health were not being communicated to those closest to the river.

Under a red alert - the highest level of threat for blue-green algae - people are advised to avoid all contact with the water, as the algae can cause gastroenteritis as well as skin and eye complaints.

Town water authorities had been notified and had upgraded their water treatment processes last night, while farmers were advised to keep their stock away from the blooms as well.

But despite yesterday's warning, large crowds flocked to the river to escape the high temperatures across the state's north.

Paul Gemmill runs the cafe at popular Thompsons Beach near Cobram, where up to 40 people were swimming in the river yesterday afternoon.

''There is no signage whatsoever to say 'beware', so unless you've seen TV or heard the radio, there's no warnings whatsoever,'' he said. ''You would think there should be signage on all major entries into the national park or state forest.''

Karen Anderson's two children were among those swimming in the river.

She said she was unaware of the alert, but felt confident that such warnings were often exaggerated.

The blooms occur when high water temperatures and high nutrient levels coincide, and despite claims the blooms are naturally occurring events, John Williams from the Wentworth Group of Concerned Scientists said poor river management was also to blame.

''We are making the river much more prone to algal blooms by the way we run it and also the nutrient load from our land use,'' he said. Mr Williams said large shallow dams along the Murray - such as Hume and Lake Mulwala - were also part of the problem.

This bloom is the second within a year for the Murray, after a major bloom derailed the peak Easter tourism period last year.

It is the latest blow to tourism in the state's north, which has already suffered this year from the new system of bushfire warnings, which encourages people to evacuate national parks on dangerous fire days. ''It affects us dramatically because we rely on tourism.

''Every time someone shouts 'bushfire' or 'drought' or 'algae' it scares people away,'' said Mr Gemmill.

It was unclear yesterday how long the bloom would last, but authorities were advising people not to eat mussels or crayfish caught in the affected regions, and to eat only fish that were thoroughly cleaned and gutted.