geology of salt creek

8/12/2019 Geology of Salt Creek

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Cretaceous age deposited about 2' million years ago are present in a band that trends northeast

to southwest

igure *. Shaded map of (ebraska showing glacial boundaries and modern streams.

across eastern (ebraska. $ost of these rocks lie west of 3ancaster County and the Salt Creek

basin however. The outcrops of bedrock that we see along Salt Creek and its tributaries in

eastern (ebraska are the tops of some of the hills that existed before they were buried beneath

sediments deposited by the glacial ice. Sandstone of the Cretaceous 4akota ormation which

underlies the marine shales and chalk capped the hills. %eneath the 4akota on the preglacial

landscape were shale and limestone of Pennsylvanian and Permian age. These rocks !5' to &''

million years old are not exposed in Wilderness Park but Permian limestone and shale can be

seen farther south along Salt Creek near #oca. #ocks of Pennsylvanian age lie beneath

Pleistocene sediments in the eastern part of the basin of Salt Creek. 0utcrops of Cretaceous

4akota sandstone which were once the tops of hills on the preglacial landscape surface can be

found near the north end of Wilderness Park as well as along /ntelope Creek and near 6ankee

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7ill cemetery. 8t also crops out in northwestern 3ancaster County where it was once 9uarried

for building stone and farther east along the Platte #iver. 4akota shale is used for brick at the

6ankee 7ill %rick Company.

igure !. 4rainage lines of the Salt Creek %asin.

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The advances of glacier ice into eastern (ebraska though buried the former land surface

beneath a thick accumulation of till gravelly sand and silt )%urchett and others *+2!,. 8t

changed completely the appearance of the land and rerouted most of the streams. The forward

margin of the ice sheet either dammed rivers that once flowed toward the east or shunted then

around the ice margin. The Platte #iver which once drained northeastward )Stanley and Wayne

*+2!, was moved into a route of what is now the %lue #iver then as the last of the glaciers that

extended into eastern (ebraska melted followed its present course eastward. 3akes formed in

every valley that was blocked by the ice dams. "lacial meltwater as well as runoff from the land

and from the mountains to the west filled the lakes to overflowing. The overflow water in the

lakes drained southeastward around the margin of the glacier eventually reaching the $ississippi

#iver and flowing into the "ulf of $exico. ine1 grained sediments11silt and clay11carried into

the lakes by both the streams that drained into them and the sediment1laden glacial meltwater11

settled out in the 9uiet water of the deep ice1marginal lakes filling the lower parts of the valleys.

The ice also expanded into the lakes leaving after it melted away a thick layer of till a massive

sediment composed of sand silt and clay as well as boulders and cobbles interbedded with the

lake muds.

The Salt Creek basin was fairly close to the maximum extent of the ice sheets that

reached the edge of the "reat Plains. 4rilling records show that the ice once reached as far west

as 6ork (ebraska )igure *,. The last glacier to enter the region reached Seward where it

shunted the %ig %lue #iver into a route along its margin. $ost of the streams in eastern

(ebraska came into existence as ice1marginal drainage lines during this final advance and

meltback which took place about :''''' years ago )Wayne *+;5,. /lthough many of


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(ebraska-s streams particularly those north of the Platte #iver mark successive positions of the

glacier margin during its disappearance a few including Salt Creek have a different orientation.

Salt Creek downstream from 3incoln is one that returned more or less to a position it had

occupied before the glaciations although many of its tributaries follow the alignment of the ice1

marginal streams. %ut its route from #oca to 3incoln and that of 0ak Creek a ma=or tributary

from the north mark one of the marginal positions of the last glacier that extended into eastern

(ebraska )igures * and !,. arther north the West ork of $aple Creek was also established

around the edge of the glacier at the same time.

>xposed in the wall of the limestone 9uarry north of #oca )igure &, is a record of two

advances of ice into the Salt Creek basin. 3ying directly on the limestone of Permian age is a

rubbly deposit left at the edge of the glacier. 8t was buried beneath a se9uence of ripple1marked

igure &. 4iagram of the east wall of the Schwarck 9uarry north of #oca

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silt beds that accumulated as a density flow in the lake that formed at the glacier margin )Stanley

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meanders )igure


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igure


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igure 5. Cross section of oxbow lake sediments exposed in bank of %eal Slough

between the location of figure < and its =unction with Salt Creek.

DChannel improvementE enlarging and straightening the channel a procedure generally

called Dchanneli@ationE began in 3incoln about *+': as a techni9ue to reduce the fre9uency and

intensity of floods in the city )/nonymous *+:5F,. rom *+'+ until about *+5' the Salt Creek

channel was enlarged and straightened eventually through /shland to the Platte #iver. This

procedure of removing the meanders shortened the channel between 3incoln and The Platte from

:+ miles to about &: miles. The shorter channel distance increased the gradient or slope of the

channel and as a result the more rapidly flowing water scoured the channel floor and banks as

the stream reestablished e9uilibriumBor DgradeE a relationship between stream velocity slope

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and sediment load. Within the reach in Wilderness Park the increased scour deepened the

channel floor as much as ! to & m.

8n its course through Wilderness Park Salt Creek flows through a channel that meanders

nearly as much as did the entire stream before it was extensively modified through 3incoln and

downstream in efforts to reduce the damage to structures caused by flooding. %ecause it has

been preserved from channeli@ation through most of the park today one can get a glimpse of the

kind of stream that drained this region before that modification took place. /s a meandering

stream that flows through fine grained sediments the current undercuts the outsides of the

meander bends leaving the alluvial sediment standing in cleanly swept steep bluffs that tend to

collapse. The lower part of the sediment accumulation is sandy and permeable so that during

episodes of high flow water infiltrates the sediment. /s the flow subsides water that has

infiltrated the sandy sediment drains back into the channel. Some of the sediment commonly

flows with it further steepening the bank. %ank failure as a debris fall or a slump often follows

)igure :,. 8f the volume of material that collapsed into the channel is large enough it may

change the position of the current and start channel migration. Throughout Wilderness Park the

banks of Salt Creek exhibit many terrace1like benches where sediment has slumped into the

channel.

8n addition to the channel modification a series of nine multipurpose reservoirs for flood

control and recreation were built on Salt Creek and several of its ma=or tributaries during the

*+:'Gs. ive of these reservoirs are on tributaries that enter Salt Creek in 3incoln downstream

from Wilderness ParkA one is on Cardwell %ranch which enters near the middle of the parkA and

three are on tributaries that enter Salt Creek upstream from the park. 0ne ma=or tributary that is

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not controlled by a reservoir =oins Salt Creek within Wilderness ParkA %eal Slough enters the

park boundary =ust north of the parking lot on W. Pioneers %lvd.

igure :. %ank collapse along Salt Creek.

%eal Slough heads in the upland near Cheney near the southeast edge of 3incoln. or

about two and a half miles it flows through farmland but downstream from 5:thStreet it passes

into an urbani@ed landscape and runoff from the residential and commercial properties becomes

much greater than it is from farmland. The stream flows through an artificially straightened

channel from 5:th

Street to the edge of the park. Where the high1velocity stormwater discharge

through the channeli@ed part of %eal Slough enters the unchanneli@ed segment at the edge of the

park it has undercut the bank and kept it swept free of the sediment that collapses into the

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channel )igure


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methods do reduce the fre9uency of flooding but they also provide a false sense of security.

/fter they are in place many people tend to believe that they are fully protected from all floods.

Thus if undeveloped land exists in the floodplain downstream from the dams it is likely to

become urbani@ed.

Through 3incoln Salt Creek really no longer has a floodplainA reduction of flooding in

the city depends on the upstream dams the enlarged channel and a levee system to prevent

excessive precipitation from flooding in the city. %ecause they are largely unmodified the flood

plain and meandering channel of Salt Creek through Wilderness Park slow the floodwater that

reaches it and provide a significant measure of natural flood protection to the City of 3incoln and

reaches downstream.

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#>>#>(C>S

*. /nonymous *+:5F The development of a program for the Salt1Wahoo watershed in (ebraska.

?(3 College of /griculture and 7ome >conomics 5< p.

!. %urchett #.#. I.7. 4rees@en >.C. #eed and ".>. Prichard *+2! . %edrock geologic map

showing thickness of overlying uaternary deposits 3incoln uadrangle and part of

(ebraska City uadrangle (ebraska and ansas ?.S. "eological Survey $iscellaneous

"eological 8nvestigations $ap 2!+ scale *K!5''''.

&. ilinska >wa *++5. Pollen study of late Pleistocene and 7olocene sediments from eastern

(ebraskaL paleoecological and paleoclimatical implications. ?(3 "eology 4ept. $S

thesis *+2 p.


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igure *. Shaded topographic map of (ebraska showing the eastward drainage of the central and

western parts of the state and the alignment of streams that mark successive positions of the ice

margin within the eastern part of the state that was glaciated )modified from Conservation and

Survey 4ivision ?(3,.

igure !. 4rainage lines of the Salt Creek basin. /lthough downstream from 3incoln Salt Creek

flows northeastwardA compare the alignment of (orth 0ak10ak Creek1Salt Creek from #oca to

3incoln with that of %ig %lue #iver and with the ice1margin positions suggested in igure *.

igure &. 4iagram of the east face of the Schwarck 9uarry near #oca showing se9uence of tills

lake sediments and loess overlying Permian limestone.

igure


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sediments are the same as those exposed in figure


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geology of salt creek

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