A coupled one-dimensional (1D) and two-dimensional (2D) hydrodynamic model for small rivers and streams on a large background area using an immersed boundary method (IBM) was developed and implemented for a simple flooding case in a previously published study. In this study, the IBM model was applied for simulations on a real geographical area, the Calcasieu Lake and its surrounding area in Southwest Louisiana. The simulation area ranges from the city of Lake Charles on the north to Gulf of Mexico on the south, and two National Wildlife Refuges (NWR) on the east and west side of Calcasieu Lake. A small change in the natural state of Calcasieu Lake, i.e., the change in the water surface elevation or flooding, can have a drastic impact on the ecosystem of its surrounding wetlands. The flooding to the wetlands is mainly through small rivers connected to the lake. In order to study the effects of coastal flooding to the wetlands, the IBM 1D–2D coupled model was implemented in the simulation package. The main purposes of this study are to: (1) determine the appropriate number of immersed boundary points to be used for this simulation and (2) test the applicability and validate the IBM model for an actual geographical region. Due to the lack of measurement data for the small rivers, the validation was conducted by comparing simulated results from IBM to the results from a non-IBM approach (i.e., manually carved rivers). Data from NOAA and USGS were used for the boundary conditions for the 2D model.