S2Urbanization, particularly in the crowded Northeast, has created a new environment. Asphalt and concrete have replaced hundreds of square miles of soil, buildings have replaced trees on much of the land, and sewers have replaced streambeds in many areas. As a result, the hydrology of this land is changing rapidly. To determine the effects of this expanding urbanization on forested watersheds, the Northeastern Forest Experiment Station has studied streamflow records of forested and urbanized watersheds. This paper is a report of that study. We consider first the urban and forest environments in relation to their differing effects on several hydrologic processes and on water quality. Second, we estimate the effects of urbanization on evapotranspiration and runoff by theoretically imposing urban conditions on a forested watershed for which climatic and streamflow records are available. Third, we describe the effects of actual progressive urbanization on peak flows, stormflows, and annual runoff of three Northeastern watersheds; and we compare peak flows, percentage of runoff, high- and low-flow intervals, and hydrologic responses between four partially urbanized and nine mostly forested watersheds. S3.