High-resolution red-green-blue (RGB) images from remote sensors, such as those carried on aircrafts, UAVs, satellites, and the International Space Station (ISS), are cost-effective data sources for real-time emergency response applications. We describe an assessment undertaken on spectral behaviors to evaluate the effectiveness of two high-resolution RGB image datasets for mapping and monitoring of floodwater extent in dense urban areas. The assessment was as part of a case study of the Calgary 2013 flood event. The input imagery included very high-resolution aerial photos and imagery acquired with the SERVIR Environmental Research and Visualization System (ISERV) carried on the ISS. The results demonstrate the complementary nature of these two RGB image sets in providing effective urban floodwater mapping for real-time response. The aerial photos with higher spatial resolution and less atmospheric effect can provide the details about the floodwater distribution; the images from ISERV-ISS can provide the temporal variation of floodwater distribution.