Random or probabilistic surveys simply refer to process of randomly selecting sites to monitor. As a result, an unbiased estimate of the condition of the resource, in this case rivers and streams, can be obtained. The process is similar to that of an opinion poll where a sample of randomly chosen individuals are asked questions about their views on a particular topic and the results from this sample are used to approximate the general opinion of the population. Results from such polls are often accompanied by a margin or error. Similarly, random survey results have an associated error term (e.g., 95% confidence interval) that, given the variability observed within the sample, indicates the reliability of the estimate. The smaller the error term, the more confidence one should have that the reported estimate is close to the actual figure for the entire population.
From 2000 to 2004, the Environmental Protection Agency (EPA) conducted a random survey of wadeable streams across the conterminous United States. This survey was designed to produce statistically-valid estimates of stream condition for the lower 48 states as well as individual estimates for three regions and nine ecoregions. A total of 17 sites were sampled in Minnesota as part of this survey. MPCA staff assisted in this survey by developing site information, contacting adjacent landowners, and making the initial visits to evaluate the suitability of each selected sample site. Data collection in Minnesota’s sample sites occurred in 2004 and was conducted by an EPA contractor. The final report summarizing the results of this survey was produced in 2006 and can be found on EPA’s Web site (see link below).
The next iteration of this national survey will include wadeable as well as non-wadeable rivers and streams. Data collection for each iteration of the survey will occur over a two-year period and will be repeated every five years (see Random Monitoring Schedule). The results of this survey will be able to assess whether the condition of wadeable streams has improved or degraded since the first survey was conducted. It is anticipated that in 2009 a total of 48 sites will be sampled in Minnesota as part of this survey. The final report summarizing the results of this current survey is scheduled to be released in 2011.
At each monitoring site, samples are collected and measurements are recorded in an effort to characterize the biological, chemical, and physical condition of the river or stream. In the wadeable streams assessment (2000-2004), macroinvertebrates were the only aquatic community that was sampled. The focus of the current rivers and streams survey is on biological assessments based on fish, aquatic macroinvertebrate, and algal communities. Other parameters that will be measured include water chemistry, nutrients, chlorophyll-a, sediment enzymes, bacteria, fish tissue, and physical habitat characteristics. Once again MPCA staff will assist with initial site evaluation and landowner contacts, but will not be responsible for data collection at these national sample sites.
While separate random surveys are currently being conducted for rivers and streams at both the national and state level, it is anticipated that these surveys will eventually serve complimentary roles or possibly be combined into a single unified effort. In order to do so, however, methods and assessment protocols of the two surveys need to be examined to explore opportunities for using the data from each survey in the analysis of the other.
Due to the design of the national survey, statistically-valid results can not be derived specifically for Minnesota. The reader is referred to the national reports for results pertaining to the two ecoregions that cover Minnesota and its neighboring states: Upper Midwest and Temperate Plains.