Stream centerlines
Small stream centerlines were developed for a dual purpose. The first was to be used to create hydrography for the Iowa DNR's statewide floodplain mapping project in accordance with the Rebuild Iowa Advisory Commission’s 120-Day report to the Governor in November of 2008. The second was to be used to conflate data from the National Hydrography Dataset (NHD) to a more accurate resolution stream network in order to be consistent with the already existing NHD framework. For this reason streams start at 24 acres of flow accumulation where there is a visible channel in the surface of a Digital Elevation Model (DEM) based on the State of Iowa’s LiDAR collection. This linework was the basis for creating hydrologically corrected DEMs that were used to determine the one square mile starting point for modeling flood plains, per FEMA standards and specifications. The linework has been collected using the State of Iowa’s Statewide LiDAR product as the main determination for flow as this surface was used when carrying out Iowa’s Statewide Flood Plain Mapping Project. In cases where LiDAR was insufficient for capture, aerial photography from various years for the State of Iowa was referenced. The NRCSs Eight-Digit Hydrologic Unit boundaries (HUC8) is the Federal Emergency Management Agencies (FEMA) specified unit of study for flood plain mapping designations. Rather than studying by county, FEMA realized that more accurate products resulted from considering an entire HUC8 and gave a more complete look at what contributed/minimized flood damage. Therefore all data was initially compiled into HUC8s, then further compiled into this statewide dataset.
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