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Water Quality Trends in Missouri Lakes

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This is the third post in a series about water quality trends in Missouri’s surface waters. The first post contained introductory material. The second post reported water quality trends in Missouri’s streams. This post looks at trends in the number of lake acres that have been assessed as supporting all intended uses vs. impaired. Missouri’s natural lakes are limited to oxbow lakes, sinkhole ponds, and open water areas in wetlands. The rest of Missouri’s lakes are man-made, ranging from small ponds to large reservoirs. This data looks only at lakes that are large enough to be “classified,” that is, qualify for protection under the federal Clean Water Act. The status of unclassified lakes is unknown.

Figure 1. Data source: Missouri Department of Natural Resources 2015.

Figure 1. Data source: Missouri Department of Natural Resources 2015.

Figure 1 shows the status of Missouri’s classified lakes. In the chart, the blue shows the number of lake acres that were fully supported. The red shows the number of acres that were impaired. The dark gray shows the number of acres that were not assessed, but which the Department did not suspect to be impaired. The light gray shows the number of acres that were not assessed, but which the Department suspected were impaired. In some years the Department listed how many acres 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 the chart, the first thing that jumps out is that the number of classified lake acres has not varied over the years to the same degree as has the number of stream miles.

Second, the Department seems to be assessing a much larger percentage of classified lakes than streams. In all years but 1, over 85% of classified lake acres were assessed.

Third, the percentage of classified lake acres assessed to be fully supported is higher than for stream miles. Across the time period, an average 67% of acres were supported for all intended uses.

Fourth, despite the generally higher level of support, the situation varied greatly across the years, from a high of 93% assessed support in 2006 to a low of 41% assessed support in 2010. In each of the three most recent reports, more than 33% of acres were assessed to be impaired for at least one use. This may be somewhat misleading, however, because in those years the Department assessed a lower percentage of lake acres than in other years.

Fifth, if one assumes that the Department’s guesses about unassessed lake acres are correct, and adds to the fully supported miles the miles for which non-support is not suspected, then the percentage of streams that are supported for all intended uses would range from 50% to 93%.

Figure 2. Data source: Missouri Department of Natural Resources 2015.

Figure 2. Data source: Missouri Department of Natural Resources 2015.

Counting only lake acres that were assessed, Figure 2 shows the percentages that were fully supported vs. impaired. The blue represents acres that were fully supported for all uses, the red those that were impaired for one or more uses. The chart shows that in only 2 years, 2006 and 2008, were a very high percentage of acres supported for all uses. In every other year, more than 25% were impaired, and in one year, 2010, 54% were impaired.

I don’t know why the number of impaired lake acres increased so much in 2010. It would make a great research project for a college student. If anybody knows the answer, please let us all know by posting a comment.

The reasons Missouri surface waters may be impaired can vary. Some reasons may be related to human causes, others to the nature of the terrain where the water body is located. I will look at the causes of impairment in 2014 in the next post.

Source

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.)

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