The sources of pollutants that impact freshwaters in the United States are thought to be nearly equally divided between point and non-point sources. The Federal Water Pollution Control Act Amendments of 1972 classify nonpoint sources in four categories: agricultural activities, forest-related activities, mining activities, and construction activities. Urban runoff is a nonpoint source that does not reach the aquatic ecosystem through a pipe. An Environ-Control, Inc. report on a study of the Delaware River estuary revealed that 40-80 percent of the total annual chemical oxygen demand and biological oxygen demand entering receiving waters from a city originated from sources other than treatment-plant discharge. The point sources primarily are toxicants (potentially hazardous chemicals), heated wastewaters, suspended solids, and degradable organics. In addition, deleterious impacts to stream biota may result from velocity changes from impoundments or once-through cooling systems for steam-electric power plants as well as channelization. This chapter will not discuss these effects but rather a strategy for mitigating them.