Category Archives: Regional Priorities

Regional priorities within the Southwestern Division

As the lake turns: The seasonal cycle of lake stratification

In the summer months, deeper lakes like Lake Eufaula will stratify because of temperature but as the fall months bring in cooler weather the layers will mix as the water temperatures equalize.
In the summer months, deeper lakes like Lake Eufaula will stratify because of temperature but as the fall months bring in cooler weather the layers will mix as the water temperatures equalize.

TULSA – During the summer months a naturally occurring phenomenon unfolds below the surface of many Tulsa District, U.S. Army Corps of Engineers lakes, as organisms compete for precious oxygen molecules. As autumn approaches, and temperatures cool, many lakes will undergo changes.

Photosynthesis and Fish

As sunlight penetrates the lake’s surface, or epilimnion, photosynthesis occurs and oxygen, created by aquatic plant life like algae, is released into the water in the form of dissolved oxygen. Surface waters also warm much more quickly than those deeper in the water column.

When fish swim through the water, their gills process this dissolved oxygen allowing them to breathe.
With so much light during the summer months, it seems natural that the water would have plenty of oxygen for fish, right? Not necessarily.

In warm water, oxygen becomes less soluble making oxygen less available for fish and other aquatic life.
Striped bass, for example, desire at least 5 milligrams of dissolved oxygen per liter of water.

When the water temperatures rise, the surface becomes less dense and oxygen molecules are less soluble, sometimes resulting in less hospitable conditions for fish, particularly when fish and other aquatic life are crowded in a limited volume of water. This can become particularly intense during nighttime hours when photosynthesis is not producing dissolved oxygen and oxygen is consumed by decomposition.

Thermal Stratification

In deeper lakes like Lake Texoma and Lake Eufaula, an event called thermal stratification occurs during the summer months. Thermal stratification is simply the development of persistent layers, characterized by dense, cold water called a hypolimnion near the bottom of the lake and the warmer, less dense epilimnion near the surface.

“In the summer, a persistent layer of warmer water containing oxygen develops to a depth of around 20 to 25 feet from the surface,” said Steve Nolen, Chief of the Tulsa District’s Natural Resources and Recreation Branch. “When the water on top warms it causes a density difference between the two layers and these layers often don’t mix.”

The hypolimnion gets little or no light, minimizing or eliminating the possibility of photosynthesis. This layer also is not subject to aeration through wind and wave action. This cold, deep layer is often oxygen-deficient and unwelcoming to fish.
“At the bottom of the lake you get what’s called an anoxic environment,” said Paul Balkenbush, environmental specialist at the Lake Texoma Project Office. “The fish won’t go down there because there is so little oxygen.”

Life Goes On

Though fish are absent, life is present in the hypolimnion in the form of bacteria feeding off decaying matter from the lake. These bacteria deplete the water of oxygen and release gases that are trapped in the hypolimnion.

“The lower layer doesn’t get oxygen and bacteria and organisms in the sediment and water column use the oxygen that is available as well as create conditions that promote release of various chemical compounds from the sediments,” said Nolen.
A relatively thin layer of water characterized by a steep temperature drop and depleted oxygen concentrations lies between the epilimnion and the hypolimnion. This layer, called the metalimnion or thermocline, is where you’ll find many fish.

If you’ve ever gone scuba diving in the summer, you’ve probably experienced a thermocline as you descend. You can’t see it but you can feel it as the temperature quickly drops.

When thermal stratification occurs, larger fish will tend to hang out in the metalimnion, where more dissolved oxygen is available and water is cooler.

Bigger Isn’t Always Better

Unfortunately for the larger fish, the oxygen in the metalimnion is limited as more fish use the available resource.

“You get what’s called a temperature/oxygen squeeze,” said Nolen. “If the dissolved oxygen falls too far hypoxia occurs and the fish get stressed, stop feeding, and sometimes die. That’s when you see fish kills.”

In such conditions, the big fish are at a disadvantage. Larger fish species generally require more oxygen to survive.

“If you get a big fat striper, it has a lot less tolerance for temperature and low oxygen, than smaller fish,” said Balkenbush. “Temperature and oxygen changes are tough on those big fish. The little fish aren’t as susceptible to oxygen depletion.”

What’s that Smell?

In Oklahoma, lake surface temperatures begin to drop in October. When the epilimnion falls to a temperature approaching that of the hypolimnion, wind mixing causes the layers to disappear as the lake “turns over” or mixes.

“When lake turnover occurs, you’ll sometimes know it by the smell,” said Nolen. “Gases, like hydrogen sulfide, are mixed in surface water and you’ll notice a rotten egg-like smell. At the same time, mineral compounds containing iron and manganese become less soluble and are adsorbed back into the sediments. Water supply users may experience brief periods of taste and odor in their finished water.”

Although lake turnover may occasionally be offensive to our olfactory senses, it’s actually good for the health of the lake as oxygen from the surface is mixed downward and nutrients from the lake bed are forced upward.

“It’s a natural process and it restores dissolved oxygen to all levels in the lake water column,” said Nolen. “It makes the entire water column once again useful for fish and other organisms that require oxygen.”


Army Corps of Engineers projects prevent $13.3 billion in flood damages during spring rains

Addicks and Barker Reservoirs, located near the intersection of I-10 and State Highway 6 in Houston, helped prevent $2.1 billion in flood damages during the recent spring rain event.
Addicks and Barker Reservoirs, located near the intersection of I-10 and State Highway 6 in Houston, helped prevent $2.1 billion in flood damages during the recent spring rain event.
Water flows over the spillway at Lewisville Lake near Dallas after heavy rains in the area in May. About 35 trillion gallons of rain fell across Texas alone in May, with heavy rains also in Oklahoma and Arkansas, putting Army Corps of Engineers reservoirs and flood risk reduction structures to the test.
Water flows over the spillway at Lewisville Lake near Dallas after heavy rains in the area in May. About 35 trillion gallons of rain fell across Texas alone in May, with heavy rains also in Oklahoma and Arkansas, putting Army Corps of Engineers reservoirs and flood risk reduction structures to the test.

U.S. Army Corps of Engineers flood risk reduction projects in the south central and southwestern United States prevented an estimated $13.3 billion in damages to local communities and infrastructure during the May-June 2015 flood event, according to recent calculations by Corps officials with the Southwestern Division in Dallas. The most damages prevented were in the greater Dallas-Fort Worth area, where the figure stood at $6.7 billion. Closely following was the greater Houston area with $6.4 billion in flood damages prevented.

“The Army Corps of Engineers flood risk reduction infrastructure—constructed, operated, and maintained with our great partners at all levels—and the benefit that it provides to our nation came to the forefront during this year’s extreme rainfall event, and our structures performed as designed,” said Brig. Gen. David C. Hill, Southwestern Division commander. “The fact that more than $6 billion in damages were prevented in both the Dallas-Fort Worth and Houston areas—the nation’s fourth and fifth largest metropolitan areas—underscore the very robust and tangible benefit this infrastructure provides, along with the other key benefits that our lakes provide throughout the region: hydropower, water supply, and recreation.”

May 2015 was the wettest month on record for both Texas and Oklahoma, and set numerous records throughout the region. Continuing rains from Tropical Storm Bill in June resulted in Army Corps of Engineers flood risk reduction reservoirs and other systems put through a rigorous test to hold the floodwaters and protect local communities and downstream areas.

The breakout for the $6.7 billion in the Dallas-Fort Worth area includes the following: $1.2 billion in damages prevented by the flood damage protection at Grapevine Lake; $2.5 billion at Lake Ray Roberts; and $2.4 billion at Lewisville Lake.

The figures for the $6.4 billion in the greater Houston area include the following: $4.3 billion in damages prevented by the Houston Flood Channel improvements (Brays Bayou and Sims Bayou) and $2.1 billion by the Buffalo Bayou reservoirs (Addicks and Barker reservoirs).

Additionally, the Arkansas River Basin projects (which include parts of Kansas, Oklahoma, and Arkansas) prevented approximately $350 million in flood damages. The Red River Basin projects (which include parts of Oklahoma, Texas, Arkansas, and Louisiana) prevented approximately $150 million in flood damages.

During this flood event, the Southwestern Division had 51 flood control lakes in flood pool and 23 in surcharge pool. Eight new pools of record were set. The Division was in an emergency operation status for two months, which was also the length of time that the McClellan-Kerr Arkansas River Navigation System was not navigable by industry. Corps projects sustained approximately $209 million in damages, much of that at its recreation sites on the lakes. The Southwestern Division covers some 2.3 million acres of public land and water across five states.

Estimating flood damages prevented is a multi-stage process that involves looking at the water level with the flood reduction project (dam or levee) in place, and where the water level would have reached if the dam or levee had not been built. Economists and hydraulic engineers looking at the damages occurring with the dam or levee in place versus no dam or levee in place calculate the estimated economic damages prevented.