July 2014

Red Rock’s Dirty Secret

With help from Mark Thompson, Red Rock Lake Association Save the Lake Committee member

The Red Rock Lake Association formed a committee that is concerned about the amount of sediment that runs into Lake Red Rock. The following article was written for the Knoxville Journal Express and the Pella Chronicle to educate about those concerns.

All rivers and streams contain sediments. When a river is stilled behind a dam, the heavier sediments in the water are no longer suspended because the velocity required to transport these materials is reduced. These sediments sink to the bottom of the impoundment. The reservoirs in central Iowa – Red Rock and Saylorville are no different. As the sediments accumulate, the reservoirs gradually lose their ability to store water for which they were built – flood storage, recreation, and for fish and wildlife use.

The rate of reservoir sedimentation depends mainly on the size of water backed up compared to the amount of sediment flowing in. A large reservoir on an extremely muddy river may rapidly lose capacity, while a small reservoir on a very clear river may take centuries to lose an appreciable amount of storage. The amount of sediment carried into a reservoir is at its highest during periods of high precipitation and associated flooding. Commonly, half of a river’s annual sediment load may be transported during only a few days of high flow due to high precipitation upstream.

Midwestern reservoirs like Red Rock and Saylorville have heavy silt loads due to the high percentage of land in their watersheds being managed for agricultural row crops. The portion of a river’s sediment load captured behind a dam is called “trap efficiency.”

This 1991 USDA-produced video about the sedimentation problems of Lake Red Rock. The year and the people involved have changed, unfortunately sedimentation is still a problem! Visit the rivergages page to check the current pool level of the reservoir.

An earlier study conducted by the Iowa State University Department of Civil Engineering sampled suspended solids (sediment) at sampling stations above, in, and downstream of Saylorville Reservoir on the Des Moines River, both before and after the reservoir became operational. The study concluded several important findings:

    1. the annual trap efficiency was dependent on the annual runoff,

    2. non-point sources were shown to be the primary contributors to water quality in the Des Moines River system, and

    3. the most important, the average annual “trap efficiency” was 90%.

It is believed by researchers that the “trap efficiency” for the Red Rock Reservoir would be similar. Many might argue that the silt loads going into Red Rock Lake should be less since Saylorville acts as a silt basin for it. However, we must understand that the Raccoon, Middle and North River basins, plus Whitebreast Creek run unimpeded into the reservoir. What does this mean? In a nutshell, this means that 90% of the sediment that flows into the reservoir, stays in the reservoir where it settles out. It is estimated that during an average day, the amount of sedimentation required to fill seven Olympic-sized swimming pools flows into Red Rock Reservoir. That is nearly 2,300 pools worth of sediment filling in the lake each year.

Most modern dams are designed so that they can afford to lose some storage capacity without impairing their performances – the part of a reservoir known as “dead storage” which lies beneath the elevation of the dam’s lowest outlet. This loss, though, can negatively affect recreation and fish and wildlife habitat. Sediments, however, do not accumulate evenly along a horizontal plane, so that some “live storage” is usually lost before the dead storage is filled. The actual process of sediment deposition is unique to every reservoir and is impossible to predict accurately. Generally, the courser, heavier sediments like sand and gravel, tend to settle out at the upper end of the reservoir or wherever water drains into the reservoir. This can form backwater deltas which gradually advance toward the dam. A perfect example of this is where Whitebreast Creek flows into the lake near Iowa Hwy 14 north of Knoxville. Lighter sediments, like silt and clay, tend to be deposited anywhere in the reservoir where water velocity is reduced to near zero. Photo below shows the Whitebreast Area north of Knoxville (the highway runs north-south here so you are looking to the SW).

It is estimated that the loss of water storage in the Red Rock conservation pool due to sedimentation is approaching fifty percent. As sediment is deposited above the water level of the conservation pool such as in White Breast Creek and in numerous bays from other smaller drainages, valuable flood storage, acres for aquatic recreation, and fish and wildlife habitat are being reduced.

Should we be concerned? Read the Iowa DNR's coverage of the sedimentation problem at Lake Red Rock in this Iowa Outdoors article.

Here are some vocabulary that may be new to you!

    • A watershed is an area of land that drains all the streams and rainfall to a common outlet such as the outflow of a reservoir, mouth of a bay, or any point along a stream channel.

    • Surface runoff is the water flow that occurs when the soil is saturated to full capacity (think of a full sponge) and excess water from rain, snowmelt, or other sources flows over the land.

    • Non-point source pollution is caused by rainfall or snowmelt moving over and through the ground. As the runoff moves, it picks up and carries away natural and human-made pollutants, finally depositing them into lakes, rivers, wetlands, coastal waters and ground waters.

    • Silt is the particles finer than sand which are carried in a suspended state in moving water.

    • Dead or inactive storage refers to water in a reservoir that cannot be drained by gravity through a dam’s outlet works, spillway, or power plant intake and can only be pumped out. Dead storage allows sediments to settle which improves water quality and also creates hydraulic head along with an area for fish during low levels.

    • Active or live storage is the portion of the reservoir that can be utilized for flood control, power production, navigation and downstream water releases. In addition, a reservoir’s flood control capacity is the amount of water it can regulate during flooding. The surcharge capacity is the capacity of the reservoir above the spillway crest that cannot be regulated.

    • A backwater delta is a part of a river in which there is little or no current. It refers either to a branch of a main river which lies alongside it and then rejoins it or to a body of water in a main river which is backed up by an obstruction such as the tide or a dam.

Check out this YouTube video that shows a unique perspective of water-transported sediment!

Ponds and Silt

Ponds are another body of water where silt becomes an issue. Marion County Park’s pond was getting shallower and shallower and the environmental education programs of fishing, studying macroinvertebrates and other organisms in the pond were drastically changing from season to season. The turbidity was quite noticeable and algal blooms were beginning to occur. Turbidity is caused by sediment and the water appears cloudy.


    • Macroinvertebrate: Organisms that lack a backbone that you can see with your eyes. These could include insects, worms, mussels, snails, crustaceans, and others.

Scroll through this student-created slideshow about the middle school field trip!

    • This Benthic Macroinvertebrate Key (opens as PDF) will helpful to have on your next water study! Benthic macroinvertebrates are the insects that comprise the most diversity of organisms. They are vital in the food chain of aquatic environments as they are important players in the processing cycling of nutrient and are major food sources for fish and other aquatic animals.

    • Iowa DNR Volunteer Water Monitoring

The original depth of Marion County Park’s pond when it was built in 1961 was 24 feet and in 2011 the depths ranged from 3 feet to 9 feet. The Conservation Board began work to alleviate the siltation issue by creating a sill pond. The purpose of the sill pond and dam is to capture the silt prior to entering the pond. The Conservation Board has completed the sill pond, drained the pond and is preparing to dredge in the Fall of 2014. The Iowa DNR sampled fishery data explains a few details about the issues of siltation.

Marion County Pond Fishery Summary from Iowa DNR Fisheries

The pond was sampled in 2011 for initial fishery data before efforts began to improve the pond. Bluegills from 3 to 6.5 inches were very abundant and would be considered stunted, or not reaching their growth potential. Angler preferred size for bluegills is greater than 7 inches. A fair population of largemouth bass are also present in good numbers and condition from 5 to 15 inches with most fish being between 8 and 11 inches. Channel catfish are present in overabundance, particularly in the 10-16 inch size, and could use some angler harvest. Common carp are also present in the pond and undesirable due to their ability to remove all submerged aquatic vegetation. The absence of aquatic vegetation reduces the number and diversity of macroinvertebrates. This reduced number of macroinvertebrates is likely the primary cause of the poor bluegill growth. No additional fish species were observed.

A significant portion of the pond’s initial water volume has been lost to sedimentation from the watershed. The shallowness of the pond allows wave action to re-suspend the sediment and nutrients causing poor water clarity and algae blooms, which is also detrimental to fish growth, especially bluegills. Ponds and lakes with high turbidity or frequent algae blooms rarely contain fish populations with good size structure and are dominated by small or stunted fish. A shallow pond with algae blooms is also more prone to summer and winter fish kills. This is due to low dissolved oxygen levels occurring during night time, calm, cloudy days, and heavy ice and snow cover when algae and decaying matter use all of the dissolved oxygen in the smaller water volume compared to a deep pond.

Check out this Google Photos "Water Quality and Sedimentation" album to learn about macroinvertebrates that can be found when there is good water quality and those that can be found as water quality declines.

Try this!

Gather and build your own water test kit to test water temperature and water turbidity, we'll help you! Think of a body of water near where you live that you can use as your 'research site.' Remember if on private property to ask permission and if in a public area to follow park rules.

Temperature: The easiest thermometer to use is one designed to float in a fish aquarium. These could be purchased where tropical fish are sold. Tape on a good length of string or cord to the top of the thermometer for easy retrieval. Talk about the importance of temperature in a habitat (remember, habitat is the place an animal can find food, water, and shelter). Practice using a thermometer at home by testing warm, hot, and cold tap water. Remember to not hold the bulb of the thermometer before you place in the water and keep in place for a minute or two for an accurate reading.

Record the temperature of the water in the following places:

    • In direct sunlight

    • In the shade

    • At the surface

    • At one foot below the surface (if you have a floating thermometer, you'll have to hold the thermometer down)

    • In moving water

    • In still water

What may cause the temperatures to vary? Now, leave the thermometer out of the water to take the air temperature. How do these temps compare? Did you know that water absorbs more heat than air for each degree of temperature change? Bodies of water change temperature more slowly and have more stable temps than land. Lakes and oceans warm adjacent land in winter and cool it in summer.

Turbidity: Gather these materials and build your own Secchi disc - large plastic lid at least 6-8 inches in diameter, black and white waterproof paint, drill or punch, large eye bolt and nut, rope, tape measure, and permanent marker. This activity uses a Secchi (pronounced seck' key) disc to measure depth of light penetration or turbidity. The greater the depth where the disc is visible, the less turbid the water.

To make a Secchi disc:

    1. Paint the lid white, when dry paint a large black X on top so you have four quadrants on the lid. Paint two opposite pieces of the pie black.

    2. Punch or drill a small hole in the middle of the X and attach an eye bolt and nut (nut on the unpainted side), then attach a string to the eye bolt.

    3. Mark off 0.5 meter or one foot increments on the string with the marker or tie knots at these points, start from where the bolt meets the disc. Your finished disc so look something like the graphic at right!

Secchi discs are best used in water that is fairly deep and slow or still. An easy place to test turbidity / use the disc is on a dock.

Gently lower the Secchi disc straight down into the water, counting the marks or knots as it is lowered until you can no longer see the black or X. Note this depth. Slowly raise the disc back up until you can just barely see the X; hold the disc there. Have someone reach down and grasp the string right at the surface of the water and hold the string there and bring the disc out of the water. Record the depth where your helper is holding the string. Add the two readings together and divide by 2. This value is a relative indicator of water clarity. Take readings from three places to compare the turbidity. You can also lower the Secchi disc all the way to the bottom to measure the total depth of the body of water.