This is the second post in a series looking at the water quality of Missouri’s lakes and streams. The first post, here, contained introductory material and an explanation of terms. This post looks at trends from 2002-2014 in the number of stream miles that have been assessed as supporting all intended uses vs. impaired. This data looks only at streams that are large enough to be “classified,” that is, qualify for protection under the federal Clean Water Act. The status of unclassified streams is unknown.
Figure 1 shows the status of Missouri’s classified streams. In the chart, the blue shows the number of miles that were fully supported. The red shows the number of miles that were impaired. The dark gray shows the number of miles that were not assessed, but which the Department did not suspect to be impaired. The light gray shows the number of miles that were not assessed, but which the Department suspected were impaired. In two of the years covered, the Department listed how many miles were not assessed, but did not say whether they were suspected to be fully supported or impaired. Those miles are shown with a hatched pattern.
(Click on chart for larger view.)
In looking at this chart, the first thing that jumps out is that for a few years the total number of classified stream miles decreased. As it seems unlikely that the topography of Missouri changes that much, I don’t know what the decrease was about. The changes were not large, and they seem to have returned to the 2002 level in the last two reports. Perhaps they were drought related, or more likely, perhaps they related to changes in reporting criteria or methods.
The second thing that jumps out is that, within classified streams, the number of miles assessed has varied widely. In 2004, 99% of the classified stream miles were assessed. In all other years, less than 67% of stream miles were assessed, and in 2002, 2012, and 2014, less than 50%.
The third thing that jumps out is that in 6 out of 7 years, less than half of Missouri total stream miles were assessed to be unimpaired for all intended uses. In 2002, 2012, and 2014, the percent assessed safe for all intended uses was 20% or less. This may be misleading, as in those years the bulk of stream miles were not assessed. Still, it leaves a gap in our knowledge about the safety of our streams.
Fourth, if one assumes that the Department’s guesses about unassessed streams are correct, and adds the fully supported miles to the miles for which non-support is not suspected, then the percentage of streams that are unimpaired for all intended uses would range from 49% to 63%.
Figure 2 shows similar information, but it includes only assessed stream miles. The blue represents stream miles supported for all uses, the red represents impaired stream miles. The chart shows that from 2006 – 2010 over 70% of all assessed stream miles were supported for all intended uses. But in 2002, 2004, 2012, and 2014, only about 50% were. Figuring out the reasons for the change is beyond this blog post, but it would be a fascinating research project for some college student. (If any body knows the answer, let us all know by posting a comment.)
I think it is hard to interpret a trend in this data – it seems to jump around from year to year. I don’t know what causes the differences. Overall, it seems that in some years half or more of Missouri’s streams were impaired for one or more of their intended uses. In other years, only 30% were impaired, but even that is a very substantial fraction.
The next post in this series will look at similar data for Missouri’s lakes.
Missouri Department of Natural Resources. 2015. Missouri Integrated Water Quality Report and Section 303(d) List, 2014. Downloaded 4/20/2016 from http://dnr.mo.gov/env/wpp/waterquality/303d/303d.htm.
(A word about the availability of the Missouri Water Quality Reports. As of 5/1/2016, reports for 2002, 2004, 2006, 2008, 2010, 2012, and 2014 are available on the Department of Natural Resources, Water Protection Website. Though the 2014 report is dated April 24, 2014, it was not available on the website until much later. The report for 2016 is not yet available, and may not be for many months.)