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Rivers
Watersheds
•  The area of land that is drained by a river is called a
watershed or drainage basin.
Large scale watersheds
Minnesota watersheds
Local watersheds
Stream ErosionThe ability of a stream to erode and transport material is
affected by many factors.
•  Velocity
•  Gradient
•  Discharge
•  Channel shape
Velocity-Speed
•  The distance that water travels in a given amount of time.
(meters/second) (m/s)
– Higher velocity=more erosion and can carry larger particles
– Slow velocity=>.27m/s (1ft/s)
•  Slowest at sides and bottom b/c friction.
Velocity
•  A young river generally has a rapid, or high velocity.
•  A Mature river has a moderate velocity.
•  An old river has a slow or sluggish velocity
Gradient
•  Also known as the slope of the river.
•  Expressed as change in elevation/distance (m/km) or (ft/mile).
•  Slope decreases as it goes from headwaters to its mouth.
Gradient
•  A young river has a steep gradient. It is also sometimes
characterized as the headwaters of a stream.
•  A mature river had a moderate gradient. This is the in-between
stage.
•  An old river has a low gradient. Imagine the mouth of the river
as it meets a lake or the ocean.
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•  An old river has a low gradient. Imagine the mouth of the river
as it meets a lake or the ocean.
Discharge
•  Described as the amount of water that passes a certain point
in a given amount of time, usually one second.
•  Measured in cubic feet per second. (ft3/s)
Discharge
•  A young river usually has smallest discharge
•  A mature river has a larger discharge.
•  An old river has the greatest amount of discharge.
Channel shape
•  The shape of a river changes with age.
A young river has a V shaped channel.
A mature river has a U shaped channel.
An old river has a |____| shaped channel.
Other factors
•  You have two streams with the same gradient, slope and
channel shape; one stream has a rocky bottom and the other
has a muddy bottom. Which will have a greater velocity and
why?
Stream Erosion
•  Hydraulic action -Streams erode by breaking up bedrock at
bottom of river.
•  Abrasion -Using sand, pebbles and boulders as “tools” by
abrasion. What shape are most rocks and pebbles in a river?
•  Dissolution -Dissolving rocks/minerals in water. (Carbonic acidCO3-2 ) limestone dissolves faster than granite.
Stream Erosion
•  A young rivers erosion usually is a downward focus. (↓)
Downcutting
•  A mature rivers erosion is focused in all directions. (←↓→)
Downcutting, lateral
•  An old rivers erosion is focused mainly on the sides of the
river. (←→) lateral
– Note how this represents the basic channel shape of rivers
as they age!!
•  All do headward erosion- channels get longer and branch out
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as they age!!
•  All do headward erosion- channels get longer and branch out
at the top.
Transport
•  The rock and material that are carried downstream by a river
are called the load.
– Dissolved load
– Suspended Load
– Bed Load
Dissolved Load
– Minerals and material in solution (dissolved) in the actual
water (e.g. Ca, Mg, CO3-2) “hard water”
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Suspended Load
– Particles of fine sand and silt.
– The velocity of the water keeps them suspended so they
don’t sink.
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Bed Load
– sand, pebbles and boulders at bottom of river (generally too
heavy). Materials are pushed, rolled, bounced along the
bottom
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Load
•  A young river can usually carry or move materials from boulder
sized to silt particles.
•  A mature river transports medium sized particles in its bed load
and suspended load.
•  An old river carries fine grained sediments in its bed load and
suspended load.
•  A flash flood can change all of this!!
Stream Deposition
Where it takes place
•  A young river deposits some in the channel.
•  A mature river deposits some sediment in the channel and on
the inside curves.
•  An old river deposits most of the sediments on the inside
curves and in the stream channel.
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curves and in the stream channel.
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Velocity and Grain size Relationship
Capacity
Competence
•  A measure of the total amount of sediments that a river can
carry.
•  The maximum size of particles that a river can carry.
•  Both depend on the velocity and discharge of a stream which
can change over time.
Stream Deposition Features
•  Delta
•  Alluvial Fans
•  Floodplain
•  Natural Levees
Delta- Alaska
•  Form at the end or Mouth of a river
•  A flat, fan shaped deposit of fine sediments from a river as it
enters a quiet body of water.
•  This is a delicate balance of erosion and deposition. Ex:
Mississippi River.
•  Named after the Greek symbol ∆ because of its shape.
Alluvial Fan- California
•  A sloped,fan shaped structure of coarse sands and gravel
sediments that are deposited where a mountain stream drains
onto a valley.
Flood Plains
•  Areas that will be covered first if the water level rises
•  The sediments here are usually fine and good for growing
crops (which is why people risk living there)
Natural Levees
•  The accumulation of deposits along the banks of a river from
overflowing or
flooding, making raised banks.
Other terms to know
•  Base level
•  Divide
•  Flash flood/flood
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•  Flash flood/flood
•  Head
•  Riparian Zone
•  River system
•  Tributary
•  Waterfall
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Base Level
•  The level of the largest body of water into which the stream
flows.
•  A stream can’t flow up against gravity!
Divide- Wind River Mtns., Wyoming
•  Ridges or elevated regions that separate watersheds.
Flash Flood
Head (Source) Headwaters of Mississippi River
•  The starting point of a river or stream.
Riparian Zone
•  Part of the watershed immediately adjacent (next) to the
stream channel
– “Green belt”
River System
•  A river and all of its tributaries.
Tributary
•  Smaller streams that flow into a larger one.
•  Ex: The Vermillion River.
Waterfalls-Minnehaha Falls
•  A steep descent of water from a height.
Stream evolution
•  As a streams load, discharge, and gradient decrease, the
erosive power of the stream decreases.
•  Over time the channel erodes and becomes wider and deeper
•  As a river evolves, it may develop curves and bends, called
meanders
Braided Streams- Alaska
•  Sediment bars between the river banks divide the water flow
into multiple channels.
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into multiple channels.
•  Large sediment load-coarse sand & gravel
•  Sand bars can move.
Meandering Stream
Oxbow lake
•  Formed by the varying speeds of water through a channel.
Eroding one side, and leaving deposits on the other side.
Human impacts on flooding
•  What are things we do to increase the likelihood of flooding?
•  What things do we do to help decrease flooding?
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