The objectives of this research were to understand and use HEC-HMS, a hydrological model developed by US Army Corps of Engineers, to develop a method to document land use changes over time in a small watershed, and to evaluate the impact of land use changes on the stream flow of Monocacy Creek, Northampton County, PA. Geospatial and land use data of the Monocacy Creek Watershed in 1958, 1986, and 2008 were collected and processed in ArcGIS. The input variables for model building in HEC-HMS were then calculated based on those GIS data. Three simulation models of the surface runoff and stream flow based on information from the three different years were then established and calibrated with the actual rainfall and discharge records. The calibrated models were then used to run the model for each land use scenario on the same rainfall event and the volume of generated runoff and the resulting stream flows are compared with one another. The results showed that as the urban development in terms of residential areas increased more than four times from 1958 to 2008, the runoff generated from the watershed area increased by almost six times from 1958 to 2008. In addition, an exponential relationship could possibly exist between the size of residential areas and stream flow discharge. Since the increase was consistent from 1958 to 1986 and from 1986 to 2008, we conclude that urban development has a significant impact on the stream flow of Monocacy Creek. Future study on the change of channel shapes and flooding patterns of Monocacy Creek can help fully assess the impact of the land use changes in the watershed.