Flood hazard in a basin depends upon the hydrological response of the upstream basin area. The upstream basin area may produce different amounts of run-off for a given rainfall based on its hydrologic response. The present communication shows the importance of drainage network characteristics in understanding the hydrologic response of a basin. The study is carried out through Geomorphic Instantaneous Unit Hydrograph analysis, wherein Horton's morphometric ratios were used to define the drainage network. A floodprone river basin in north Bihar plains has been selected as a study area. The study shows that the length ratio (RL) significantly influences the hydrologic response of a river basin.