Because the focus of River Network is the health of rivers, the approach described in this manual emphasizes water-based sources of pollution. However, just as pollution knows no political boundaries, it also does not remain confined to any one type of environmental media. Regardless of whether initial pollution discharges are to be found in the air, soil, surface water or groundwater, one can expect to see that contamination show up in other places further “downstream.” Despite our emphasis on water and watersheds, the methods described in this guide are equally pertinent to investigating environmental health problems that have their source in air or soil pollution.