The modified topographic index (TIm) based on digital elevation models (DEMs) was employed to delineate flood-prone areas in Mahanadi basin, India. TIm and flood inundation maps were compared to obtain the threshold (τ) beyond which the area is assumed to be inundated by flood and the exponent of the TIm. Scale dependence was also investigated to evaluate the sensitiveness of spatial resolution of the DEMs. DEMs of five resolutions, namely, ASTER global, SRTM, GMTED2010 (30 arc-seconds), GMTED 2010 (15 arc-seconds), and GMTED 2010 (7.5 arc-seconds) were used and ASTER global was preferred due to its low error compared to the remainder. Flood frequency analysis was conducted to obtain the relationship between flood-prone areas and flood magnitude. It was observed that (i) the exponent in the TIm showed little variation, (ii) τ is reduced with reducing spatial resolution of the DEM, and (iii) error is also reduced as the DEMs' resolution is reduced.