The return of the urban stream For most of the twentieth century, city planners treated small rivers as a nuisance to be buried. Culverting a stream freed the land above it for roads and housing and removed an obvious drowning risk. The practice is now being reversed in dozens of cities, a process known as daylighting. The arguments for it are rarely nostalgic: an open channel carries flood water more slowly than a pipe of the same capacity, and a pipe that fails fails completely, whereas an open channel that overtops still drains. Daylighting is expensive, and the land it consumes is usually the most valuable a city owns. Where it has been done, however, the flood defence it provides has generally cost less over thirty years than enlarging the culvert it replaced. According to the passage, why were urban streams originally culverted?
