Regional Probabilistic Stream Assessments
Probabilistic monitoring is the sampling of randomly selected stream sites. These stream sites on rivers and streams throughout a basin or region are selected in an unbiased manner by a computer program. Each stream site in a basin or region of interest has an equal probability or chance of being selected for monitoring.
Typically, most water monitoring done by DEQ over the years has been at targeted, non-randomly selected sites. Most of these targeted sites are at bridges on larger rivers. Data from targeted sites are important in telling us about compliance with water quality standards, tracking general water quality trends at a particular location, and identifying pollution problems. However, data from targeted sites are only representative of conditions at that location. It is not appropriate to extrapolate data from targeted sites to describe regional stream conditions. To answer larger, regional questions, such as “How good is the water quality in the Willamette basin?” requires a probabilistic monitoring approach. This approach is very similar to how public opinion polls are conducted for large groups of people where the data from a relatively small, representative, random sample are used to describe the characteristics of a much larger population.
Although DEQ has used probabilistic monitoring approach to assess Oregon streams and rivers since 1994, at this time there has been no comprehensive probabilistic water quality assessment for the state of Oregon to answer regional or state-wide water quality questions. Our probabilistic monitoring has been to answer local regional questions and has usually been funded through short-term grants.
Two DEQ programs using probabilistic monitoring approach have been the Oregon Plan for Salmon and Watersheds (Oregon Plan) and EPA’s Environmental Monitoring and Assessment Program (EMAP). Oregon Plan is a program that looks at the condition of salmon stream habitat for selected salmon species. EMAP is an EPA funded program that looks at regional stream conditions in selected basins around the state. A more detailed description of these monitoring programs follows.