Runoff Pollution in the Lower Fox River Basin
This photo shows the mouth of the Fox River in Green Bay after a June 2004 rain storm. The water is chocolate-milk brown because of sediments that are being carried into the bay by the surface runoff from across the basin.
According to the Wisconsin Department of Natural Resources, urban and rural nonpoint source pollution (also called runoff pollution) is the leading cause of water quality problems in Wisconsin. In rural areas, runoff pollution comes primarily from farm fields and barnyards. In urban areas, runoff pollution comes mainly from roads, lawns, parking lots, and construction sites. The main pollutants of concern in the Lower Fox River Basin are sediments and phosphorus.
Sediments
Sediments are particles of soil, small bits of organic material, particles of rubber from tires, and other small, solid materials that are found on the ground. When these sediments are carried to lakes and rivers in runoff, they can harm fish and other aquatic organisms, lead to the need for dredging to sustain navigation, and contribute to algae and weed growth.
Phosphorus
Nutrients are the elements – mainly nitrogen, phosphorus, and potassium – that plants need to grow. They are found both as molecules attached to soil particles and as molecules dissolved in water. Runoff can carry nutrients to waterways either by transporting sediments or by transporting nutrients in their dissolved form. The nutrient that causes the most concern for surface water bodies is phosphorus. Excess phosphorus can cause algae blooms, which in turn leads to water that smells, looks unappealing, requires greater treatment to use as drinking water, and harms fish and other aquatic organisms.
One common problem associated with algae blooms is the depletion of oxygen in the water. Algae consume oxygen after they die and begin to decay. If enough oxygen is used up, the fish in the water can no longer respirate and they will either move to other, healthier parts of a waterbody or they will die. The amount of dissolved oxygen in a waterbody is an important indicator of water quality.
For more information: Brown Water, Green Weeds (300 Kb, pdf) 
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What can I do to prevent runoff pollution?
