What do these rivers have in common with the Platte River in Nebraska?
- The Brahmaputra-Jamuna River and the lower Yellow River Asia,
- The Tagliamento River in Italy,
- The Narew River in Poland,
- The Tanana River in Alaska, or
- Medano Creek in Colorado
They are all BRAIDED RIVERS, a geological phenomena you will learn about in this EarthCache.
BRAIDED RIVERS
Many streams become faster flowing after heavy rains or the spring melting of winter ice and snow. The fast flowing water can carry large amounts of sand, gravel, and small stones. The flow of water will slow in sections of the stream passing through wide valleys with little downward slope, allowing the sediment to settle to the streambed. This sediment will start to block the stream, slowing the water even more until enough settles to the bottom to form sandbanks or shoals that block the stream. The stream will then overflow its banks and make new channels. The stream may split into many separate channels that join together only to split apart again farther downstream. The many interconnected channels of braided streams, when viewed from above, look like strands a braid.
A braided stream is a stream that has small channel capacity with respect to its sediment supply. Basically, there is not sufficient room within the stream valley to deposit all of the sediment and therefore, small islands of sediment form within the stream giving it a braided appearance. Braided streams generally occur at high gradients.
When the river is carrying vast amounts of eroded material as bedload. as water levels fall and the energy decreases the river drops its coarsest material. the rapid deposition of its coarsest material begins to block the main channel causing the river to divide into smaller channels that seek to find the way through the obstructing channel.
Braided rivers, as distinct from meandering rivers, occur when a threshold level of sediment load or slope is reached. Geologically speaking, an increase in sediment load will over time increase the slope of the river, so these two conditions can be considered synonymous; and, consequently, a variation of slope can model a variation in sediment load. A threshold slope was experimentally determined to be 0.016 (ft/ft) for a 0.15 cu ft/s (0.0042 m3/s) stream with poorly sorted coarse sand.[1] Any slope over this threshold created a braided stream, while any slope under the threshold created a meandering stream or— for very low slopes—a straight channel. So the main controlling factor on river development is the amount of sediment that the river carries; once a given system crosses a threshold value for sediment load, it will convert from a meandering system to a braided system. Also important to channel development is the proportion of suspended load sediment to bed load. An increase in suspended sediment allowed for the deposition of fine erosion-resistant material on the inside of a curve, which accentuated the curve and in some instances caused a river to shift from a braided to a meandering profile. The channels and braid bars are usually highly mobile, with the river layout often changing significantly during flood events. Channels move sideways via differential velocity: On the outside of a curve, deeper, swift water picks up sediment (usually gravel or larger stones), which is re-deposited in slow-moving water on the inside of a bend.
Conditions associated with braided channel formation include:
- an abundant supply of sediment
- high stream gradient
- rapid and frequent variations in water discharge[4]
- erodible banks
- a steep channel gradient
However, the critical factor that determines whether a stream will meander or braid is bank erodibility. A stream with cohesive banks that are resistant to erosion will form narrow, deep, meandering channels, whereas a stream with highly erodible banks will form wide, shallow channels, inhibiting helical flow and resulting in the formation of braided channels.
REQUIREMENTS FOR LOGGING
Do not attempt this EarthCache when the river is at flood stage.
Visit the posted coordinates which are located south of I-80 Exit 305 on S. Alda Rd. Parking is located on the NW side of the Platte River Bridge at a Sandhill Crane overlook. From this overlook you will be able to see the distinguishing characteristics of the “BRAIDED” PLATTE RIVER.
In order to claim a FIND on this EarthCache you much complete the following. Do not post your answers on the LOG. E-mail your answers to me.
1. Observe the Platte River to the East and West to see how the River is divided by many channels and sand bars. WHAT CHARACTERISTICS OF A BRAIDED RIVER DO YOU SEE?
2. HOW MUCH SEDIMENT IS IN A WATER SAMPLE?
Bring a collection jar with a tight fitting lid. Go to the edge of the river and notice the abundance of suspended material in the water. The Platte River isn’t clear like a meandering mountain stream. It is full of suspended sediment. Fill up your jar with water from the river and seal the lid on tight. Set the jar aside at let it clear over the next couple of hours. Answer the question by measuring the amount of sediment in comparison to the amount of water collected. Answer example: There was 1 tablespoon of Sediment in 1 pint of water. OR, try using a coffee filter. Pour water through the filter to see how much sediment you collect in a qt or gal of water.
3. WHAT IS THE SIZE OF SEDIMENT at the “Sample Coordinates”. By observation of by digging close to the water’s edge, describe the size of material on the land surface.
4. WHAT’S CAUSING EROSIAN near the parking lot at the “Erosion Coordinates”?
Notice the size of material in the erosion channel. Do you see any river stone? There is plenty of broken concrete in this area, not to be confused with river stone.
4a. WHATS THE LARGEST RIVER STONE EXPOSED FROM EROSIAN that you see in the erosion channel.
5. Read the Sign in the Parking area about the Sand Hill Cranes. Fossilized cranes have been discovered or uncovered in deposits of the Platte River. HOW OLD ARE THESE DEPOSITS?
OPTIONAL
6. Take a picture of the Platte showing the Braided section to the East-downstream or West-upstream.
GEOLOGICAL INFORMATION ABOUT THE PLATTE RIVER
The Platte River of south-central Nebraska was studied at three scales to place the river in its geological context and to trace its evolution through geologic time. At the largest scale the Elm Creek West and the Newark 7.5 minute quadrangles were mapped. These quadrangles are located just west and just east of Kearney and serve to illustrate the main geomorphic elements of the present Platte River Valley. The central elements of the quadrangles are the Platte River channels, islands, and bottomlands, which are flanked by terraces that step up away from the river to the north and south. Significant other elements of the landscape are eolian sand and loess deposits. The geologic maps are supplemented by topographic profiles of the mapped terraces and graphical representations of subsurface units in test wells that occur within the quadrangles.
An intermediate-scale study consisted of examining descriptions of well cuttings in a 17 county area in south-central Nebraska, which includes the Platte River Valley, and building a database of information about sediment lithology and thickness. The wells penetrated a sequence of gravel, sand, silt, and clay beds from the ground surface to the top of the subsurface Tertiary Ogallala Group or Cretaceous formations. The sequence consists of Pliocene-, Pleistocene-, and Holocene-age strata that document the deposition of a veneer of alluvium by late Tertiary and Quaternary streams intermixed with and overlain by wind-blown loess. Various isopleth and structure maps illustrate the distribution and alluvial architecture of the sedimentary sequence, and support the interpretation of former positions of the Platte River.
A regional-scale study consisted of documenting the geologic history of the Front Range and adjacent mountains and depositional areas east of the mountains in Colorado, Wyoming, Kansas, and Nebraska from the end of the Cretaceous Period, about 65 million years ago, to the present. The structural and sedimentary history of the region is outlined, and a series of paleogeographic maps shows the development of the drainage system in the east-central Rocky Mountains and adjacent Plains. Ancestral South Platte, North Platte, and Laramie Rivers are recognized as early as the late Eocene, although the South Platte probably flowed to the southeast from the mountain front at that time. Deposits of the North Platte River are recognized on the west side of the Medicine Bow Mountains of Wyoming in the Miocene, and the presence of distinctive rock clasts indicates that the Laramie River flowed from the North Park area of Colorado northeast across a filled Laramie Basin and the Laramie Range of southeastern Wyoming in the Miocene. The present drainage system developed in the late Miocene to the Pliocene and included the capture and diversion of the South Platte River into its present channel. The combined North and South Platte Rivers deposited gravel and sand across Nebraska and flowed southeast from Kearney, Nebraska through the middle to late Pleistocene. Within the past 25,000 years the Platte River below Kearney was captured and diverted into its present course and confined there by bounding valley walls of loess.
REFERENCES
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