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Scientific Investigations Report 2007–5186

Scientific Investigations Report 2007–5186

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Water-resource managers in the Pacific Northwest need information regarding the status of and trends in regional water-quality conditions. This information is useful in watershed planning, such as Total Maximum Daily Load (TMDL) development required by the Clean Water Act. Many streams in the Pacific Northwest have levels of nutrients (nitrogen and phosphorus) and suspended sediment that are elevated enough to cause adverse ecological effects, including excessive aquatic plant growth, reduced water clarity, reduced stream dissolved oxygen, and altered stream habitat. In addition, water-resource managers need information on the important sources of nutrients and sediment delivered to streams. This is especially true with regard to TMDL development, because source reduction is the main tool available to lower in-stream pollutant loads. There also is a need to quantify the relative contributions from tributaries and direct anthropogenic discharges (wastewater-treatment plants and industrial facilities) to the total nutrient and sediment load in the major rivers of the Pacific Northwest and, ultimately, to the Pacific Ocean.

To determine the extent of nutrient and sediment concerns in our Nation’s streams, analysis of the information collected during Cycle I of the U.S. Geological Survey’s (USGS) National Water-Quality Assessment Program (NAWQA) basin-specific water-quality studies in the United States, along with other Federal, State, and local agency studies, were assessed regionally (Gilliom and others, 2001). The addition of other agency data greatly extended our ability to understand contaminant occurrence and transport, especially during times of declining funding. This study is the culmination of a similar effort to assess the trends and the transport of total nitrogen (TN), total phosphorus (TP), and suspended sediment (SS), to provide water-quality managers in the Pacific Northwest the information they need to effectively develop watershed improvement plans. The work done for this study can be a resource to support future investigations by regional water-quality managers and will be the foundation for a more detailed and rigorous regional water-quality modeling effort by the USGS that will assess both pollutant transport and load allocation.

Purpose and Scope

This report summarizes (1) patterns in nutrient and suspended-sediment loads and yields and (2) trends in concentrations and loads in relation to landscape, management practices, and climate in the Columbia River and Puget Sound Basins during an 11-year period, water years (WYs) 1993 through 2003. (A water year begins on October 1 of the previous calendar year and ends on September 30). The report focuses on:

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