© 2015 ASCE.The Cache Creek Settling Basin (CCSB) is a 14.5 square-kilometer sediment retention basin designed to trap sediment and reduce sediment loads into the Yolo Bypass. The basin, and the 13-kilometer stretch of leveed creek from the United States Geological Survey (USGS) Cache Creek at Yolo gauge (11452500) to the upstream boundary of the basin, were engineered to safely conduct flows up to 850 cubic meters per second with approximately 1-meter of freeboard. The sole purpose of the basin is to retain sediment and protect floodway capacity of the Yolo Bypass. As a retention basin, the CCSB functions by spreading incoming flow from Cache Creek over heterogeneous terrain, consequently decreasing velocity and increasing residence time within the basin. Flows discharge from the CCSB via an uncontrolled overflow weir, and a low-flow culvert, such that large outflows are not managed. Simulating the hydrodynamics of the basin, focusing on water surface elevation and inundation extent, is a vital initial step in evaluating the ability of the basin to protect floodway capacity downstream. Two-dimensional unsteady flow modeling of an extent of Cache Creek from the USGS Yolo gauge through the CCSB is presented with a focus on calibration to confidently model inundation area.