Rivers near Dhaka, Bangladesh, are biologically dead and especially toxic during dry seasons, a water resource expert said Saturday.

"The rivers around Dhaka have too little oxygen for the survival of aquatic life," Umme Kulsum Navera, assistant professor of Water Resource Engineering of Bangladesh University of Engineering and Technology, told the news service IRIN.

Oxygen levels increase during the monsoons but not enough to allow for a healthy aquatic environment. Specialists warn the rivers cannot be cleaned.

The Water Resource Engineering department researched the rivers and reported they learned some invertebrates and small organisms come to life in the rivers when water flow increases. In the dry season these organisms disappear from four major rivers that encircle the city.

One of the rivers is the Buriganga, which has one of the largest river ports in the world, the Sadarghat terminal.

The Buriganga does not support any fish, the experts said.