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A Toronto architectural firm is being lauded this week for its part in rehabilitating what was once a toxic landfill in the middle of a Saudi Arabian oasis.

On Tuesday, Moriyama & Teshima Planners won one of five Aga Khan awards for architecture. The awards were established in 1977 by the Aga Khan, the 49th hereditary imam of the Shia Ismaili Muslims, to "enhance the understanding and appreciation of Islamic culture as expressed through architecture."

The award recognizes Moriyama & Teshima Planners' work on the Wadi Hanifa (or Hanifa Valley). It is the longest valley near the capital of Riyadh and part of a natural water drainage area for a region of more than 4,000 square kilometres.

However, starting in the 1970s, development pressures in Riyadh severely damaged the watershed.

"Wadi Hanifa was Riyadh's dump and its sewer," George Stockton, president of Moriyama & Teshima Planners, said in a release. "What we wanted to do was to bring Wadi Hanifa back to life. So after decades of neglect, we are now seeing this amazing rebirth and transformation of Wadi Hanifa as a naturalized park system."

In 2001, Riyadh's authorities launched a plan to restore the valley, enlisting the help of the Toronto firm.

After extensive cleanup, dams were constructed to regulate water flow, biological treatment plants built to purify the water, and recreational areas built to encourage people to visit the area again.

The award jury lauded the project for restoring the watershed's ability to clean contaminated water, mediate flooding, provide habitats for biodiversity and create opportunities for recreation.

"It shows how a major natural phenomenon, which, through the course of urbanization, became a litter-strewn and dangerous place — a scar on the face of the capital city — can be transformed by sensitive planning attentive to social values and imaginative infrastructure driven landscape solutions," the jury wrote.