Wet Weather and Water Quality
We had record rainfall this year, with most counties registering at least 14 inches above average between February and July. Rainfall in six unfortunate counties was more than 2 feet above normal for the period of February through July. Consequently, lake levels throughout the state were also higher than normal this year. What follows is a closer look at the water quality impacts of this spring’s flooding.
Table Rock Lake dam in 2008 with all flood gates open. David Casaletto photo.
What we expected
Water clarity values are generally lower when lake levels are high, thanks to sediments suspended in the runoff. As the brown, turbid water sits in a lake, particles settle out. The longer the water “resides” in the lake, the more settling will occur. With increased rainfall, however, water runs over the spillway or through the turbines, and the “residence time” is decreased considerably. As a result, the particles don’t have time to settle out and the lake remains turbid. When rains subside and the particles finally settle out, the water will clear up. At that point there will be increased sunlight penetration into the water, creating perfect conditions for algae in the lake; nutrients (from the runoff), and light. Water clarity should rapidly decrease again as the algal cells multiply. When this happens, the once brown lake will turn green.
What we found – Table Rock
Most Table Rock volunteers were unable to access their sites in April due to high water. For some, their docks were inaccessible and for others the boat ramps (or even the parking lots) were under water. Fortunately, some volunteers were able to collect April samples. Volunteers showed their ingenuity by accessing their docks via shuttle boats and, in one case, by using a ferry constructed out of a pallet and dock foam.
Figure 1. 2007 vs. 2008 Secchi values at Table Rock Lake, Site 14
For Table Rock sites where the April sampling occurred, water clarity was reduced considerably compared to previous years. For example, Secchi data from Site 14 (main channel, just below Indian Creek) show that there was 8 feet less water clarity in April 2008 vs. April 2007 (Fig 1). These data coincide with a lake level difference of roughly 15 feet. Comparing the April through July Secchi depths from 2008 to those of 1992 through 2007 (Fig. 2) shows that sites closer to the dam were most likely to exhibit reduced water clarity in the first half of the 2008 season.
Turbid water caused by the early rainfall might have traveled through the lake as a “muddy mass.” This mass would have flowed quickly through the upper portion of the lake, slowing as it neared the dam, where it might sit for a considerable time before either the sediments settled out or the muddy water exited the lake through the turbines. The longer the muddy water sits in the lake, the more likely it is that the turbid condition will be recorded by a volunteer.
Figure 2. 2008 vs. long-term mean Secchi values for Table Rock Lake main channel sites.
What we found - statewide
Declines in water clarity were inconsistent across the state in 2008. Using data from the Statewide Lake Assessment Project (see the Fall 2004 Waterline for more information about that project), we compared average 2008 Secchi readings with available long term means (Fig. 3). Data from May through August show no statewide trend in water clarity reduction. In fact, several lakes had better than average water clarity in 2008. The inconsistent responses this season may be attributable the variability of the rainfall patterns during the sampling season, land use patterns within the watersheds, timing of sample collection or residence times of the lakes sampled.
Figure 3. Long-term average vs. 2008 Secchi values for 18 Missouri lakes monitored by the Statewide Lake Assessment Project.
Soon the remaining LMVP samples will be gathered and analysis of the late summer values will begin. Then we’ll see what story the entire data set tells. The data from the second half of the sampling season could bring the averages closer to normal, reducing the long-term impact of this spring’s rain on the LMVP data set.