Projects by Discipline / Water Resources Engineering

DePuy Spring Creek Floodplain and Floodway Evaluation

Hydrosphere evaluated a recent draft detailed flood study with respect to its regulatory impact on existing development. The DePuy Spring Creek on the DePuy Ranch is one of several well-known and long standing "spring creek" type fishing sites tributary to the Yellowstone River in the Paradise Valley at the north entrance to Yellowstone National Park near Livingston, Montana.

The ranch, which originally contained a facility that raised fish for food and sport, has become a world-renown trophy cold-water fishery. Large floodplain edge limestone springs create fertile water quality and strong baseflow, resulting in a year-round quality fishing experience. While it remains a working ranch, the fishing experience is at the heart of the viable business. When it was discovered that most of the ranch, including the spring creek, was preliminarily mapped within the regulatory floodplain and floodway and that such mapping had significant regulatory impact on the property, the DePuy family became involved in the floodplain mapping process. The combination of new mapping and county/state regulations would have the effect of severely constraining existing and future uses of the ranch.

Hydrosphere reviewed the technical and mapping elements of the detailed flood study, which had been performed by the U.S.G.S. for the State of Montana. The review revealed that many areas of dry land (isolated by floodwater) were not reflected in the mapping and that the floodway depth and velocity criteria were being met at the floodplain edge by the hydraulically disconnected DePuy Spring Creek, rather than the Yellowstone River. This resulted in a much wider floodway than would result from a "rise criteria" based floodway. The need for the regulations to recognize pre-existing uses and to accommodate structures (warming, lunch and weather day use shelters) as a valid floodplain fringe use was identified. These and other related findings were presented to the State for further consideration in developing the flood maps and regulations that apply to this unique reach of the Yellowstone River.