In order to make room for the factories of the Industrial Revolution, emerging megacities such as London had planners conceive and build an extensive system of brick tunnels which allowed rivers and creeks to disappear beneath the streets, camouflaging an often elaborate network of tunnels that still carry effluent from homes to larger rivers and treatment facilities.

The ancient Romans practiced the same routine of covering up curvy rivers to fix the disruption of traffic and permanently smother odors from flowing waste matter, but that legacy has almost guaranteed most large urban centres have a handful of significant buried waterways which haven’t seen light in decades, if not centuries.

Writer / director Catherine Bacle and producer Katarina Soukup intertwine the city sagas of London, Yonkers, Brescia, and Seoul to show four very distinct case studies and their respective attempts to deal with the inevitable problem of flooding. There's also the issue of cities denying its citizens contact with the waterways and the natural flora & fauna which originally convinced settlers to build cities by the riverbanks. Montreal, and especially Toronto, are treated as cities in need of a serious rethink, as the increase construction of densely packed condos will stress an ill-prepared infrastructure largely built during the Victorian era.

Local conservationists, activists, architects, and a sliver of politicians are on hand to dissect the various options considered or rejected by their municipalities, while the nitty-gritty details are conveyed by ‘drainers’ – urban explorers who (illegally in most cities) venture into sewers to investigate, photograph (with stunning lighting and exposure effects) and map the hidden worlds that lie beneath buildings, parking lots, and major intersections.

The four international case studies bear a quick rundown: London’s already begun test projects of creating standing ponds that act as natural reservoirs to offset destructive street flooding; Northern Italy’s Brescia has officially recognized the local drainers who have thus far given 10,000 tourists an underground history of the city going back to 5 A.D.; Yonkers, New York, have ‘daylighted’ (excavated and exposed) the mouth of the Saw Mill River to create a green space in a neighbourhood plagued by shuttered businesses; and in Seoul, South Korea, a network of overpasses were torn down to restore the Cheonggyecheon River.

Seoul’s restoration project is the most controversial due to the displacement of local small businesses and the restored river not being wholly self-sufficient, but Bacle and Soukup deliver their messages through ongoing contrasts to show the before, the after, and the current states in each of the six cities, and it’s likely Lost Rivers will remain a timely essay, even a decade after its release.

Extracts from an audience Q&A at Toronto's Bloor Hot Docs Cinema with producer Soukup and Lost Rivers Toronto founder Helen Mills are available at Big Head Amusements.

© 2013 Mark R. Hasan