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A stream is a body of water that carries rock particles and dissolved ions and flows down slope along a clearly defined path, called a channel . Thus, streams may vary in width from a few centimeters to several kilometers. Streams.
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A stream is a body of water that carries rock particles and dissolved ions and flows down slope along a clearly defined path, called a channel. Thus, streams may vary in width from a few centimeters to several kilometers. Streams
Streams carry most of the water that goes from the land to the sea, and thus are an important part of the water cycle. • Streams carry billions of tons of sediment to lower elevations, and thus are one of the main transporting mediums in the production of sedimentary rocks. • Streams carry dissolved ions (minerals), the products of chemical weathering, into the oceans and thus make the sea salty. • Streams are a major part of the erosional process, working in conjunction with weathering and mass wasting. • Streams are the most influential factor in sculpting the physical earth.
The Basics • Stream Flow • Water makes its way to the sea under the influence of gravity. • The time it takes depends on the stream’s velocity. • Velocity is determined by three things • Gradient • Shape • Discharge
Gradient • The slope of a stream channel • Gradient is the vertical drop of a stream over a specified distance in miles. • Some portions of the Mississippi have a gradient of 10 cm per km.
Shape of a Stream’s Channel • The shape and roughness of a channel affects the amount of friction the water will encounter. • Smooth channels offer little friction and fast water. • Rough channels offer high friction and slower water.
Discharge • A stream’s discharge is the volume of water flowing past a point in a specified time. • Measured in cubic meters per second. • The Amazon discharges 207,600 m3 of water per second, 12 times more than the Mississippi.
Changes from Upstream to Downstream • Gradient is steep upstream (start in hills or mountains) • Velocity and discharge is great downstream (smooth stream bed and high pressure from upstream water)
Straight Channels • Rare • Low Velocity • Bars = piles of sediment. Water works its way around the bars. Velocity and erosion are greatest at banks (sides of channel)
Latin word for WANDERING • Velocity is highest at outer parts of meander (cutbank) • Erosion happens at cutbank • Velocity is lowest at inner parts of meander (point bar) • Depositioin happens at point bar • This continual erosion and deposition causes the channel to move and bend across a river valley. • Creates a flood plane, or area that a river floods. Meandering Channels
Braided Channels • Streams with highly variable discharge and easily eroded banks. • Bars get exposed during low discharge and produce a braided channel • Usually happens at deltas.
Deltas • Stream enters standing body of water. • Velocity drops and sediment is deposited. • They build outward from coastline. • Can only survive in low current and velocity.
Work of Streams • Erosion – the general removal of rock and soil. • Streams transport their load of sediment in 3 ways • In solution (dissolved load) • In suspension (suspended load) • Rolling along the bottom (bed load)
Dissolved Load • Most of a stream’s dissolved load is brought by ground water. • Expressed as parts of dissolved mineral per million. • The average for the world’s rivers is between 115 – 120 ppm.
Suspended Load • Visible suspended sediment carried by a stream (sand, silt, and clay). • Usually constitutes a streams largest load.
Bed Load • Sediment too large to be carried in suspension. • These large particles move along the bottom of the stream. • Some roll • Some hop (saltation) • Grinding action of the bed load, abrasion, aids in erosion.
A stream’s competence is a measurement of the maximum sized particle a stream can carry. The faster a stream’s velocity, the larger its competence. A stream’s capacity is the maximum load it can carry. The capacity is related to its discharge. The greater volume in a stream, the greater capacity it will have. Competence vs. Capacity
Deposition • As a stream slows down, its capacity and competence decrease. • Particles begin to settle depending on their size. • Particles of similar size are deposited together (sorting).
The Stream Cycle • Over time rivers shape and reshape the land. Just like karst topography, there are stages of river development. They are: • Youth • Maturity • Old Age • Rejuvenation
Youth • V-Shaped Valley • Rapids • Waterfalls • No Flood Plain • Somewhat straight channel • Steep gradient • Valley Being Deepened • Drainage is poor therefore swamps and lakes MAY form but not always
Maturity • V-Shaped Valley • Beginnings of Flood Plain • Increased Meanders • Gradient begins to reduce • Valleys stop deepening • Divides become rounded
Old Age • Land worn to nearly flat surface • No downward cutting taking place • Very low gradient • Rivers meander across extremely wide, flat flood plains • Natural levees may form • Oxbow lakes form
Rejuvenation • Uplift of Land (Lowering of Sea Level, sometimes greater stream flow) causes stream to speed up and cut deeper. • Stream valley takes on youthful characteristics but retains features of older stages as well. • Can happen at any point in the cycle.
