Letters to the Pilot

Thursday, August 22, 2002

Where did the silt go?


What has happened to all of the silt that is supposed to be at the bottom of Storm Lake? Recent newspaper reports mention that the dredge crew on Storm Lake is having difficulty finding enough silt to make dredging efficient. Indeed, if a great portion of the lake has only 1 foot or less of silt on the bottom, then this flies in the face of reports that Storm Lake once had a 60-foot deep channel, or was once 22 feet deep. It is difficult to believe that the lake filled in with sand and gravel and not silt over the years. Perhaps that is exactly what has has happened over the years, but in an indirect manner. Let's assume that the original depth of Storm Lake was 22 feet with a combination rock, sand, gravel and clay bottom. The clay probably formed from compacted silt over the years. Wave action washed some of the sand and gravel into the silt, and a new layer of sand, gravel and clay were formed over the original bottom.

Today we may be looking at a sand, gravel and clay bottom that is under 1 foot of silt. Below the present sand, gravel and clay layer may be another layer of hard silt or clay. In other words, we may have stratified layers of silt, clay, sand and gravel over a great portion of the lake. Perhaps if the lake had been dredged when silt first settled over the original bottom, then we wouldn't have this stratified layer to contend with today. We may have to live with the fact that we have lost the original depth of the lake. The only thing that we can now do is to try and maintain the depth that we have down to the present layer of sand, gravel and clay.

According to a map that was published by the Iowa Conservation Commission in 1972, there were six pockets in the lake that were dredged to a depth of 10 to 14 feet. Today, these pockets might possibly hold the deepest silt accumulations in the lake and represent the depth of the "new" lake bottom. Three of the pockets are located in the west area of the current dredging project, and have probably been cleaned out. Using the 1972 map as a guide, the remaining three pockets can be located and dredged during the current state project. Whether 1 foot or 10 feet of silt remain on the bottom of the lake, any expanded dredging project should concentrate on removing silt down to the rock, sand, gravel and clay level, and stop at that point. The rock, sand and gravel are excellent water filtering agents and should remain in the lake. Let's not shake the dredge to pieces by trying to dredge through hard clay, sand and gravel. Let's just bite the bullet and call it the "new" lake bottom, and try to maintain this new bottom the best way that we can.

Future expanded dredging efforts should focus on removing the deepest area of silt first and then finish the project by removing the remaining silt. The goal should be to remove as much silt that is economically possible down to the rock, gravel, sand and clay layer of the lake.

Further efforts should also continue to be made to reduce the amount of silt flowing into the lake from Little Storm Lake.

Mike Brecher,

Storm Lake