Because of state budget pressure, 14 USGS stream gauging sites supported by the Iowa Department of Natural Resources (IDNR) are at risk of shutdown. USGS stream gauges are critical to the state, federal, nonprofit, and private organizations that sustainably manage Iowa’s water resources. The value and associated costs of the gauges are primarily related to the USGS’s maintenance of rating curves—the relationships between water surface elevation and stream flow. Rating curves allow the USGS to estimate stream flow for any measured water surface elevation at a gauging site. The process is labor intensive, requiring USGS staff to visit each site every six weeks, as well as during flood events, to directly measure stream flows.

While stream rating based on direct measurements is the most accurate method used to estimate continuous stream flows, less expensive alternatives are available when direct measurements are cost-prohibitive. The Iowa Flood Center (IFC) proposed investigating the feasibility and relative accuracy of these methods, giving specific attention to seven at-risk gauging sites (listed above) identified by the National Weather Service (NWS) as important to its flood forecasting mission.

The project has two phases. First, the IFC compiled and compared rating curve data at USGS stream gauges from USGS, IFC, and NFIE sources to evaluate the practical application of currently available simulation-based rating curves to stream flow forecasting. Second, the IFC performed comprehensive surveys of the priority sites, including topographic, bathymetric, stream velocity, and drone surveys, and developed and evaluated detailed one- and two-dimensional numerical models. The IFC used the models to assess rating curve improvements resulting from the incorporation of geometric detail and improved flow physics. They also compared the model results to existing rating curve data, using effective USGS rating curves as a baseline.