Water Quality Long-Term Trends in Streams and Rivers 1971-2007

Table 1. Summary of Long-term Trend Analysis (1971 – 2007) by Stream

King County has been routinely collecting water samples in streams and rivers since the 1970s in an effort to protect the water resources in our area. Water quality data used in this trend analysis include a variety of conventional parameters: temperature, dissolved oxygen, turbidity, total dissolved solids, pH, conductivity, and nutrients (ortho-phosphate, total phosphate, ammonia-nitrogen, nitrate+nitrite- nitrogen, total nitrogen). This trend analysis evaluated data from 1971 through 2007 and included only sites that were active in 2007 and have been monitored for at least five years. For more information about the County’s Stream Monitoring Program, go to the program history page.

The results of this long-term trend analysis are summarized in Table 1 which can be viewed by clicking on the table to the right. The direction of the arrow for each parameter indicates if the statistically significant trend was increasing or decreasing (P<0.10). An “=” means there was no significant change over the time period included in the analysis. Whether or not these trends indicate an improvement or a decline in water quality depends upon the parameter and is identified by color. For example, a decrease in water temperature is good for aquatic life (colored blue), where a decrease in dissolved oxygen is not (colored tan). For more information about the various parameters listed, check out the freshwater glossary of terms.

Though the period of record for each stream varies, it can be interesting to look at the streams collectively and note trends for King County streams on a regional basis. Figure 1 below illustrates the number of streams that had increasing/decreasing trends for each parameter. Improvements in water quality on a regional scale are indicated by decreasing nutrients (with the exception of total phosphorus), fecal coliform bacteria, and total suspended solids for the majority of streams. The large number of streams with increases in total phosphorus, conductivity, and water temperatures, along with decreasing dissolved oxygen concentrations indicate declines in water quality that need to be investigated.

Figure 1. Increasing and Decreasing Trends by Parameter for King County Streams Long-term Trend Analysis (1971 – 2007).

Results of the Long-term Trend Analysis for sites in the Ambient Stream Monitoring Program are available as an Adobe Acrobat pdf. You will need Adobe Acrobat to view the document. If you do not have this software, you can download a free copy of Adobe© Acrobat© Reader.