Water-surface elevation data and flood and floodway boundaries for the upper Yellowstone River, Montana

Abstract

The upper Yellowstone River in south-central Montana is an important source of irrigation water and a blue-ribbon trout stream. In addition, an increasing number of homes are being built in the scenic upper Yellowstone River valley. In 1996, severe flooding caused substantial channel and bank erosion, particularly just upstream from Livingston (fig. 1). Following the flood, numerous projects were completed to stabilize the banks. In 1997, a severe flood again caused channel and bank erosion. Although previously constructed stabilization projects decreased erosion in some locations, they were thought to have contributed to increased erosion in 1997 at upstream or downstream locations, and future bank-stabilization projects, therefore, became controversial. To study the effects of bank stabilization on the dynamic upper Yellowstone River, a task force was appointed by the Governor of Montana. The Upper Yellowstone River Task Force initiated a cumulative-effects study of the upper Yellowstone River. The main objective of the cumulative-effects study was to gather information from a wide range of scientific investigations in the basin and to use the scientific information as a basis for assessing the long-term, cumulative effects of proposed streambank-stabilization projects. The U.S. Geological Survey (USGS), in cooperation with Montana Departments of Transportation (MDT) and Natural Resources and Conservation (MDNRC), Park Conservation District, and the U.S. Army Corps of Engineers (USACE), investigated the hydraulic characteristics of the upper Yellowstone River as part of the cumulative-effects study. The investigation of hydraulic characteristics had two major objectives: (1) determination of flood and floodway boundaries for parts of the upper Yellowstone River and (2) simulation of sediment transport for a part of the upper Yellowstone River. This report presents water-surface elevation data required to meet objective 1 and shows the mapped flood and floodway boundaries. The mapped flood boundaries show the extent of flooding from flood discharges having recurrence intervals of 500 and 100 years. The mapped floodway boundaries show the area within the boundaries of the 100-year flood discharge that is reserved for the passage of flood flows under Montana Administrative Rule 36.15.502 (1995). The hydrologic analysis is based on recorded annual-peak discharge data from 1890 through 1998 for two USGS streamflow-gaging stations. The hydraulic analysis is based on channel- and bridge-geometry data collected between 1999- 2001. The hydraulic analysis and resulting flood mapping were more detailed for the lower study reach from Carter Bridge upstream to Point of Rocks bridge than for the upper study reach from Point of Rocks bridge upstream to Gardiner.