Kigali is a city of hills, and when it rains — which it does intensely, twice a year — water rushes off those hills in torrents. For most of the 2000s, that water had nowhere to go. The valley-bottom wetlands that once absorbed runoff had been drained and built on as the city expanded rapidly after reconstruction. Flash flooding became routine, washing out roads and inundating low-income settlements built precisely in the flood-prone zones that wealthier residents had vacated.

The Kigali Green City Wetland Restoration Program, a collaboration between the Rwanda Environment Management Authority and UN-Habitat, began compulsorily acquiring and clearing inappropriately zoned valley-bottom developments in 2021 — compensating displaced residents with plots on higher ground. What lay beneath the concrete was not dead soil but dormant wetland seed banks that began re-establishing vegetation within months of drainage infrastructure being removed.

Across 40 square kilometres of restored valley wetlands, papyrus, water hyacinth, and riparian forest have re-established naturally. Hydrology data shows peak flood flows during the 2024 heavy rain season were 70 percent lower than 2020 levels, preventing an estimated $180 million in infrastructure damage.

The cooling effect has surprised even the project's architects. Daytime surface temperatures in restored wetland corridors average 3.2 degrees Celsius lower than in adjacent built areas, an effect driven by evapotranspiration from the dense vegetation. In a city increasingly challenged by heat stress, the wetlands have become natural air conditioning.

"We built on our sponges and then wondered why we were drowning," said REMA Director-General Dr. Chantal Nziyomaze. "We are simply returning to what was already there."