STREAMS & RIVERS Chapter 6

REVIEW…. The Water Cycle

6.1 Streams and Rivers River Systems (structure) Tributary- a stream that runs into another stream or river River system- a river and all of its tributaries

Drainage basin, or watershed- all the land that drains into the river either directly or through its tributaries ex) Mississippi River system Divide- high land that separates one drainage basin from another Ex) Continental Divide in the Rocky Mountains

Characteristics Velocity- distance that water travels in a given amount of time Related to the amount of energy that the water has Erosion occurs more quickly Other characteristics affect the velocity: Gradient, Discharge, and Channel Characteristics Gradient- steepness of the slope of a stream or river Steep at its source; gradual at sea level

Characteristics continued… Discharge- the amount, or volume, of water that passes a certain point in a given amount of time Not constant over length of a river Increases downstream as tributaries add more water Not constant year-round- increased precipitation/melting snow Channel- the path through which the water flows in a stream or river Shallow, winding stream with many boulders has contact with surface area that causes it to slow down by friction Straight channel that is wide and deep has less surface area in contact with water, so velocity is greater Velocity is greatest at the top center of the stream When it curves, zone of max. speed shifts to the outer bank SEE FIGURE 3 (p.160)

Different types of Stream Channels

Base Level Base Level = lowest point to which a stream can erode its channel. Base level is usually the level at which the mouth of a stream enters the ocean or the next body of water. Temporary Base Level vs. Ultimate Base Level Temporary = lake Ultimate = sea level A stream in a broad, flat-bottomed valley that is near its base level often develops many meanders.

Warmup Quiz What is the lowest point to which a stream’s water can erode the soil? What is the distance water travels in a given amount of time? A wide, deep, smooth river will have a faster or slower velocity? When a stream curves, the Zone of Maximum Speed shifts where? A shallow, narrow river will have a larger or smaller discharge?

6.2 Stream Erosion and Deposition Deposition- the process by which materials are deposited Occurs because the river or stream no longer has enough energy to transport them

Erosion Running water wears down Earth’s surface by breaking up bedrock and by removing eroded rock and soil materials Breaks up materials by mechanical means Abrasion mainly; in time, creates rounded boulders, pebbles, and sand grains from the cutting tools

Stream Flow & Erosion There is more stream velocity in the middle/top of the stream Erosion will be greater in the middle of the straight stream (increase depth) Deposition will occur on the stream/river banks where the channel turns and the velocity of the waterway drops SEE FIGURE 3 (p.160)

Erosion & Deposition Features in an Old Stream/River

Erosion cont… Potholes- deep oval or circular basins formed when water in a river develops small whirlpools, and materials grind potholes in the bedrock

Plunge pool- a basin that has been worn away at the base of a waterfall by the action of falling water Dunn River Falls Chemical weathering consists of dissolving soluble minerals Calcite is dissolved, which may form pits and holes in the riverbed Widens existing cracks and holes

Transported Material Load- the eroded rock and soil materials that are transported downstream by a river Suspension- a state in which rock materials carried by a river are stirred up and kept from sinking by the turbulence of stream flow Materials are heavier than water, but the turbulence keeps them stirred up The faster a stream flows, the more turbulent and muddy it becomes Bedload- sand, pebbles, and boulders that are moved along the bed of a stream and that are too heavy to be carried in suspension

Transporting Materials Two measures are used to describe the ability of a stream to transport materials Competence- a measure that describes the maximum size of the particles a stream can carry Capacity- a measure of the total amount of sediment a stream can carry Depend on the velocity and discharge Vary along a stream and change throughout the year High velocity + High discharge = large amount of sediment and larger sizes of sediment particles

Stream Deposition River will deposit part of its load when either velocity or discharge decreases Velocity may decrease if channel widens or river meets an obstruction (curving bank/rock outcrop) Speed decreases at the inside of the curve and when the river empties into the sea- deposits the most material Discharge and velocity increase during a flood, then decrease as flood waters subside

Depositional Features Delta- a fan-shaped deposit that forms when a river flows into a quiet or large body of water River water comes to almost a standstill at a delta Distributaries- branches formed by rivers flowing over its delta Responsible for delta’s shape Formation is delicate balance between deposition and erosion

Alluvial fan- fan-shaped deposit May form when a steep mountain stream meets dry, level land at the base of a mountain Differs from a delta Deposit is formed on land, not in water Sediments are coarse sands and gravels rather than fine silt and clay Surface is sloping, not flat like that of a delta

Stream Valleys Narrow valleys narrow V-shaped valley shows that the stream’s primary work has been down cutting toward base level Rapids and waterfalls Wide valleys Downward erosion is less dominant More energy is directed side to side Floodplain- the flat, low-lying portion of a stream valley subject to periodic flooding

Linville Gorge Carved out of the bedrock by the river forming a river valley.

Floodplains Streams that flow on floodplains move in meanders Cutoffs- shorter channel segments Oxbow lakes- abandoned bend of a river/stream

Oxbow Lake Formation  Meander forms (because the stream is close to base level)  Far side of curves erode faster, inching closer & closer together.  Curves join, and water moves past the bypass loop  Deposition occurs at the ends of the loop (because velocity drops), cutting off the oxbow lake.

Floods Most caused by rapid spring snow melt or storms that bring heavy rains over a large region Ex: Mississippi River in 1993 Measures to control flooding Artificial Levees- earthen mounds built on the banks of the river Flood-control dam- store floodwater and then let it out slowly Limits on floodplain development- minimizing the development on floodplains

Warmup Quiz In a stream meander, where will the deposition occur... inside or outside curve? Faster or slower water will erode the banks more? Where will you find more waterfalls… in a V- shaped or a wide valley? Streams that are in a floodplain should have more or less meanders? Name one way we can control flooding.