Between 1972 and 1994, the US Army Corps of Engineers undertook five studies in New England evaluating the benefits and costs of protecting natural valley storage (NVS) areas—natural reservoirs—for flood mitigation. Only along the Charles River did benefits outweigh costs. Analysis of the studies finds that the costs of large-scale land acquisition will often exceed the sole benefits of avoided flood damages. To generate net benefits, there must be significant amounts of NVS lands still undeveloped, development pressure on those lands, and downstream areas that would sustain large damages. The NVS studies also raised questions of whether the Corps should be involved in land acquisition, and whether regulating land use could substitute for purchasing land. Note, these findings do not apply to other forms of natural flood-risk reduction, such as levee setbacks, green infrastructure for stormwater management, and multi-purpose projects, which have different economic and institutional contexts.