Streams

Stream Change

-At any point in a stream, width, depth, and slope adjust to meet the discharge needs. -Changes in water volume or sediment grain size cause the channel to change too. -Discharge decreases downstream causing changes in width and depth. -Velocity increases downstream. -Sediment size decreases downstream.

Human Effects

-Dams increase slope causing increased erosion downstream and increased deposition upstream. -Urban developments alter hydrological pathways causing increased run off and erosion.

Shear Stress

-Force not velocity is exerted upon a particle to move it. -Force increases with depth and the slope of a stream. -Shear force must overcome the resisting forces of friction and cohesion that keep the particles in place.

Oxbow Lakes

-Formed when cut banks (outside bank) of a windy river erode toward one another. -When they meet they form an Oxbow lake. -Form on floodplains.

Floodplains

-Low-relief areas adjacent to streams that inundate during floods. -Shifting channels make flood plains, overbank flooding builds up floodplains by depositing thin sheets of sediment forming natural levees. -Floodplain is either Bedload that accumulated in a shifting channel or suspended load deposited when rivers spill over their banks.

Stress vs Particle Size

-More shear stress does NOT move larger particles. -The smallest particles take as much shear force to dislodge as the largest due to large cohesive forces.

Water

-Moves via the Hydrologic Cycle. Drainage Basin: the area from which a single stream or river and its tributaries drains all of the water. Tributaries: small river or stream that flows in to a larger river or stream; a branch of the river. Discharge: the amount of water that flows through a channel (cross section area x velocity = ..... cubic meters/second).

Streams

-Streams provide water and deposit fertile soil for agriculture. -They are pathways for commerce and trade. -They also flood, erode, and sculpt the landscape. -Streams are everywhere and many cities are adjacent to them.

Sediment

-Streams through physical forces erode and loosen particles surrounding the stream. -Streams also gain sediment via particles sliding down down slopes into streams. -Streams also erode their banks. -The particles a stream carries are termed its sediment load. -Vegetation catches sediment and reduces erosion. -Dissolved sediments are known as the hidden load. -Picking up and moving sediments requires work and expends energy. -Potential energy of the water is changed to kinetic energy on a down slope.

Stream Formed Landscapes

-Terraces: indicate downcutting and filling of valleys. -Bedload deposits: when well above the current floodplain they indicate the stream was at a higher elevation. -Climate change, tectonic processes, and fluctuations of sea level drive the imbalances that rivers respond to by creating terraces. -Stream pathways are determined by the rocks they erode, some rocks are harder to erode than others causing water to naturally flow around them.

Sediment Transport

Bedload- large grains that cannot be picked up, but still are able to be moved. Form dunes and cross-bedding. Suspended load- small sediment grains that mix with the flowing water are transported above the bed, rarely touching the bottom. Stream Power- is the ability of a stream to do work. -Measured by multiplying shear force with the average velocity. -Excess power causes erosion. -Insufficient power causes deposition.

Channel Patterns

Meandering(Suspended particles dominant): a single main channel that has various loops and/or curves down its length. -Curves as water velocity is faster away from banks as friction slows it down on banks. This causes erosion increasing depth on one side of the stream. Braided (Bedload abundant): a series of small channels that are interwoven into a larger whole. (Channels are often a combination of both)

Deposition

Occurs when stream loses: -Shear stress. -Velocity. -Discharge. When stream decreases rapidly in depth to 0m an alluvial fan is formed. When velocity suddenly decreases due to movement of sediments into quieter bodies of water a delta is formed.

Characteristic that Determine Channel Pattern

Slope: -A meandering channel is longer and thus has lower average slope = lower power = smaller suspended particles. -Straight channels are short and on steep slopes = more power = bigger sediment. Banks: •Bank erodibility influences channel pattern. •Easily eroded material tends to make a wider, shallower stream, leading to a braided pattern. •Where the banks are more stable because of clay or vegetation, the channel tends to be narrow, deep, and sinuous (curvy due to erosion and deposition of opposing banks).

Stream Types

Alluvial: stream flowing over alluvium, sediment that the river has transported and deposited. Bedrock: Develops when stream erodes down by abrasion to resistant bedrock.