Highway construction often causes substantial adverse environmental effects, both during and after the construction. To assess the construction impact on surroundings and mitigate its influences, environmental research is often conducted before the construction. This book focuses on GIS-based hydrological modeling of US I-99 Highway construction watersheds. Recent GIS-based hydrological studies are mostly applied to large, natural watersheds. However, the watersheds in this research are small and their topographies are severely disturbed by construction. There are a few difficulties in applying the GIS theory in the research. Several improvements were made to apply GIS-based watershed model using Watershed Modeling System. Further, a new model - Highway Watershed Model was developed, which uses a new type of unit hydrograph - Linear Exponential Unit Hydrograph (LEUH). This book provides theories and tools in GIS application in hydrologic engineering, and should be especially useful to professionals in water environmental assessment.