Runoff is surface water effluent, usually from precipitation but may be from human activities such as irrigation, that moves too quickly to be absorbed into the ground. It flows down contour gradients to enter stream and river systems, carrying with it anything light enough to be borne in the volume of water, which may be light after a small rain or tremendous in the wake of a storm, when even large boulders and trees get swept up in the runoff. When runoff travels over deforested or unplanted agricultural lands, it carries away large quantities of topsoil . Runoff from agricultural areas often carries heavy doses of biocides, fertilizers, and other nutrients, which can lead to eutrophication when introduced into aquatic systems.