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 Water Cycle

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)

Meandering Channels 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.

Competence vs. Capacity 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.

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

Yellowstone River

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

Oxbow Lakes

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.

                                                                                     Rejuvenation of an old-age landscape.

                                                                                     Rejuvenation of an early mature landscape.