Ghost rivers beneath cities: a major blind spot of flood-risk governance

Abstract As cities expand over floodplains, the streams crossing them are occasionally buried; yet these ghost rivers keep their hydrological function, generating runoff and flash floods that endanger the populations above. For the first time, we located and inventoried 50 ghost rivers entombed during twentieth-century urbanisation of Spain's Mediterranean coast, a climate-change and tourism hotspot repeatedly struck by catastrophic floods. Three-quarters of these streams lie beneath coastal cities and resorts; the remainder beneath intensive agriculture. Most (64%) fall outside the official flood-risk register, an exclusion that does not track physical hazard (catchment morphometry and soil sealing). Moreover, the unrecognised catchments receive more, not less, extreme rainfall. This registration pattern instead reflects the social geography of the coast: among 377,625 residents of these catchments, the poorer are captured in the map while the wealthier remain administratively invisible. This structural blind spot in urban flood governance has potential implications far beyond the Mediterranean.