People adversely affect watersheds when they dump pollutants on the ground, down sinks and in toilets; use harmful chemicals in activities such as washing their car or doing laundry; or treat plants and gardens with herbicides, pesticides and fertilizers. People protect watersheds when they conserve water, use hardy plants not requiring chemical additives, recycle waste, use absorptive materials such as gravel for paving land to reduce runoff and reduce driving, and pick up waste deposited by their pets.
A watershed is any land area that drains water into a common outlet such as a drainage ditch, stream, river or aquifer. Colloquially it is often used to designate the outlet itself or the area immediately surrounding the outlet. Since all land is part of a watershed, any activity that pollutes land pollutes the outlet to which runoff from the land flows. In addition, any activity that prevents runoff, such as using absorptive paving, colloquially speaking protects the "watershed," or more technically, protects the watershed's outlet.
Many sources provide information on how people can protect watersheds. Some of these resources are The Nature Conservancy, with its website Nature.org, and the Water Science School located on the U.S. Geological Survey's website at USGS.gov.