Archive for Wednesday, August 11, 1993

93 WILL LEAVE BEHIND MANY IMPRINTS

August 11, 1993

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Though the flood of 1993 has receded from most of Kansas, this summer's heavy rains and flooding will have a long-term impact.

Some of the results of the floods are readily apparent, say geologists at the Kansas Geological Survey, based at Kansas University. But others are less obvious.

For example, many of the rivers and streams that flooded may have cut new channels as they sought new paths. Those channel changes are apparent as rivers recede. But it is less obvious that the channels of those rivers may be deeper. Fast-moving stream water during floods may have scoured the bottom of rivers, at least temporarily moving out silt and sand and deepening the channels.

Rains also filled many of the large reservoirs in eastern Kansas. Others in western Kansas, such as Webster, Kirwin and Cedar Bluffs reservoirs, are not full but contain more water than they have in several years. Even if precipitation amounts return to normal, water levels in those lakes should stay elevated for some time, perhaps several years.

Increased precipitation also has affected ground water, the underground water found in the pore space of rock formations called aquifers. The movement of precipitation from the surface down into aquifers is called recharge.

"There has been a lot of recharge in parts of central Kansas where rains were heavy and aquifers are closer to the surface," said Robert Worth Buddemeier, chief of the Survey's geohydrology section. Aquifers in western Kansas, such as the Ogallala Formation, are generally much deeper and the recharge will probably be less.

"Where water tables are deep, there is normally a time lag between precipitation and actual recharge, except in certain locations such as along streams," Buddemeier said. "In western Kansas, we might not see the full effects of a major rain on ground water for years or even decades."

However, the increased precipitation may have an indirect impact on ground-water levels. With heavy rains, irrigators pump less water, and that lack of pumping may allow water levels in aquifers to recover and rise slightly, or at least fall more slowly.

Increased moisture affects more than aquifers, lakes and rivers. Soils saturated with water are far more prone to erosion.

"Gravity may cause water-saturated soils to slump," said Survey geologist James R. McCauley. "Along steep ditches next to roads in northeastern Kansas, for example, soil and rock have been torn away from slopes."

Sinkholes also have appeared with the increased rain, McCauley said. In places where there are underground void spaces, either natural or man-made, the wet ground above the void becomes much heavier. When rock layers above the void can no longer support the additional weight, they may collapse, creating a sinkhole at the surface.

The wet soil also affects foundations of buildings. Wet soils often expand, putting pressure on concrete foundations, pressure that is occasionally severe enough to crack foundations or even cause them to collapse.

McCauley said that the silt deposited by flood waters might present a problem once dry weather returns. Flooded rivers regularly drop fine-grained sediment -- such as clay, sand and silt -- along with larger cobbles and boulders where currents were especially strong.

"When dry weather returns, these new layers of silt and sand will be susceptible to blowing," McCauley said.

Finally, the floods may prove to be a boon to bone hunters. For many years, fossil collectors have picked up the remains of Ice Age mammals from sandbars and banks along the Kansas River in northeastern Kansas. The high, fast water will no doubt tear away additional soil and sand, say geologists, and may expose fossils for the first time since the animals died, about 15,000 years ago.

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