Streams & Floods:
The Geology of Running Water
• Earth is unique in that it is the only
place in our solar system that contains
streams of running water.
– Mars may have had streams in the
past.
• Flood – an event during which the
volume of water in a stream/river
exceeds the streams normal limits.
• Runoff – Water that flows on the
land’s surface
– About 10% of Earth’s water is runoff
• Reach - Any length of a stream/river
channel
• Meander – A broad curving reach
• Meteoric Water - Water that is
derived from rain or snow (weatherrelated).
Possible water-related features on Mars
• All of the water is part of the
hydrologic cycle
– Water starts out in the
atmosphere, falls as rain/snow,
is collected into streams, and
eventually empties into oceans.
– Ocean water evaporates and the
cycle repeats.
Surface Water & Groundwater
• Water Table – The boundary defining the subsurface realm where water completely
fills cracks and pores
• Surface Water - All water on the surface (lakes, rivers, etc…)
• Groundwater – All water that is stored below the ground in soil and rock.
Forming Streams
• Running water begins as Sheetwash, a
film of water a few mm thick. (like
water on a road)
• Sheetwash erodes its substrate (the
solid base layer it rests upon)
• The rate of erosion depends on
velocity, strength of substrate, and
amount of vegetation (strength of
substrate)
• Eventually the sheetwash forms a
channel in the weaker portions of the
substrate and the channel is eroded
deeper with time by Downcutting,
eventually forming a stream.
• As flow increases, Headward Erosion
(i.e. erosion in an upstream direction)
occurs because flow is faster at the
entry to the channel.
• With passing time, other Tributaries
may link together to a main Trunk
Stream.
Drainage Networks
• Interconnecting
tributaries form a
Drainage Network.
• They come in several
flavors:
Drainage Basins and Divides
• The area drained by a system of tributaries is called a Drainage Basin,
Catchment, or Watershed.
• A high ridge that separates one water shed from another is called a Divide.
Divides and Watersheds in the U.S.
• Watersheds exist in
a variety of scales.
– Tiny tributaries.
– Continental rivers.
• Large watersheds…
– Feed large rivers.
– Section continents.
• Continental divides
separate flow to
different oceans.
Permanent vs. Ephemeral Streams
• Some streams flow all year,
and some do not.
• Permanent Stream – A
stream that has a base that is
below the water table and so
it flows all year
– Usually found in temperate
climates
• Ephemeral Stream – A
stream that only flows after
rainfall events. The water
eventually flows into the
ground.
– Found in arid climates
• A dry ephemeral stream bed
is caled a Wash
Stream Discharge
• Geologists measure the amount of water that a stream carries by measuring its
discharge.
• Discharge – The volume of water that passes through an imaginary cross section
across a stream. Expressed at some volume per time e.g. meters3/second
• Discharge varies widely
•
•
•
•
•
•
Amazon ~ 200,000 m3/s
– largest discharge on
Earth
– ~15% of total runoff on
Earth
Congo ~ 40,000 m3/s
Mississippi ~ 17,000 m3/s
In arid regions discharge
decreases away from source
In temperate regions
discharge increases away
from source
Can also vary due to
seasons, or human activity
Stream Velocity
• The average velocity of a stream is
difficult to measure because flow in
streams is complex.
• Friction along the sides and bottom of
stream channels slows the water’s
velocity
• The smaller the Wetted Perimeter
(assuming the cross sectional area is
the same), the faster the water can
flow.
• Streams with meanders will have the
fastest velocity and deepest depth at
the outer banks (Thalwegs) of the
meanders.
• Crescent shaped Point Bars may
form on the inner banks of meanders.
• Complex
interactions with
underwater
boulders and the
stream banks cause
Turbulent Flow,
i.e. flow that is
complex and not
all in the same
direction
Turbulent Flow
Erosional Processes
• Streamflow does work.
– The energy imparted to
streamflow is derived
from gravity.
– Streams do work by
converting potential to
kinetic energy.
• Erosion is maximized
during floods.
– Large water volumes.
– High water velocities.
– Abundant sediment.
Water and Erosion
• The kinetic energy of moving
water can cause erosion in four
main ways.
• Scouring – running water can
remove loose fragments
• Breaking and Lifting –
running water can lift blocks
out of a material
• Abrasion – pure (clean) water
has little erosive effect, but
sand-laden water acts like
sandpaper and grinds away the
channel wall.
– This is how potholes are carved
• Dissolution – Running water
can dissolve soluble minerals.
Running water eroded these channel
walls in northern Arizona
Transportation and Sediment Load
• Sediment Load – The amount of sediment that a stream carries. It consists of
• Dissolved Load – ions floating in the water solution
• Suspended Load – sediment (silt & clay) that is suspended (no settling) within the
stream
• Bed Load – Larger particles that bounce or roll along the stream floor.
– Moves by Saltation, bouncing an rolling along the stream floor.
Deposition – When Streams Lose Their Loads
• Competence – the maximum particle
size that a stream can transport.
– Depends on stream velocity
• Capacity – the total quantity of
sediment that a stream can carry.
– Depends on competence and discharge
• Rivers will deposit their sediment loads
when particles can no longer be
transported.
• Fast-moving streams deposit coarse
sediment (pebbles & boulders) and
slow-moving streams deposit fine
grained sediment (fine sand/mud).
• Because the stream gradient, or slope
of the stream, may change at times
along a stream’s path, sediment tends
to get sorted.
• Basically, any time a stream slows
down, it deposits some of its sediment.
Longitudinal Changes
• The character of a stream
changes with flow distance.
Near the headwater source
– Gradient is steep.
– Sediments are coarse (high
competence).
– Channels are straight and rocky.
– Capacity is low
• Channel is narrow
• Discharge is low
Longitudinal Changes
• The character of a stream changes with flow distance.
– Towards the mouth*… *(where a river enters the ocean)
•
•
•
•
Gradient flattens.
Higher discharges. (Channel is wider)
Smaller grain sizes typical (low competence).
Channels carve broad meander belts.
Longitudinal Stream Profile
Base Level
• There is a depth below
which a stream cannot
cut. This is called the
Base Level.
• Base levels can be
local (e.g. a lake) but
all streams have the
ultimate base level of
sea level.
• Local base levels can
be removed over long
periods of time
Stream Valleys and Canyons
• If a stream downcuts faster
than erosion of the channel
walls, it will form narrow
Slot Canyons (e.g. Zion).
• If a stream downcuts
slowly, erosion of the
channel walls will form a
V-Shaped Valley (e.g.
New river).
• If a stream cuts through
rocks of contrasting
strengths (sandstone vs.
shale) it may form a valley
with a stair-step shape
(e.g. Grand Canyon).
Stream Terraces
• If base levels change or if
discharge changes a stream can
form multiple Terraces within
an alluvium-filled valley.
Waterfalls and Rapids
• Rapids form where water flows over boulders or where a stream’s
gradient increases
• Waterfalls form when a streams gradient becomes very steep, possibly
due to a resistant layer at its base (Niagara Falls).
• Waterfalls slowly change due to headward erosion. A good example
of this is Niagara Falls…
Niagara Falls
Headward Erosion at Niagara Falls
• Geologists
studied the
headward
erosion of
Niagara Falls
and determined
that the fall is
eroded back
towards Lake
Erie by ~1 m /
year
• Now, it is about
half that because
some water is
diverted for
hydro-electric
power.
• In ~60,000 yrs,
the falls will
erode all the way
back to Lake
Erie
Alluvial Fans
• Alluvial Fans form where a fast-moving mountain stream emerges
from a canyon at the range front onto an open plain.
• The water was once confined to a narrow channel can now spread out
• Because of this its flow slows and it deposits its sediment load in a fan
shape.
• Common in arid
Environments with
much topography
• Typically
consists of sand &
cobbles
• Rock Types:
Arkose/Sandstone
Breccia
The Badwater fan in Death Valley
Braided Streams
• In some locations, streams carry abundant coarse sediment during storms but cannot
carry this sediment during normal flow.
• During normal flow the sediment is deposited and the stream divides into numerous
meandering channels forming a Braided Stream.
• Because of the large amount of sediment that is deposited,
braided streams cannot
cut deep channels
(loose sediment is weak).
• Common to
glacially-fed streams
A braided stream
In Alaska
Meanders
• Where rivers flow over relatively flat land comprised of a relatively
soft substrate, natural variations in the substrate strength and the
velocity of the water will cause the channel to begin to form curved
segments called meanders.
• Once the meander sweeps through ~180° the meander neck, may
continue to be eroded by the cut bank.
• Eventually the river’s cut bank may erode through the meander neck,
leaving an abandoned, meander called an Oxbow Lake.
TIME
Flood Plains & Levees
• A meandering
stream occupies only
a small portion of a
greater flood plain,
which is inundated
during a large flood
event.
• Water spilling out
from the edges of a
river forms a
natural levee.
• The levee can
sometimes grow so
high that the river
bottom is higher
than the flood plain.
Deltas
• When rivers enter open water such as an ocean, they divide into
multiple channels called distributaries.
• Greek historians originally termed these deltas because the Nile river
delta is roughly shaped like the Greek letter delta. Deltas can have
many shapes.
The Mississippi river delta
The Nile river delta
Midstream Bars & Avulsion
Why do distributaries form?
• When a river enters standing water, the water in the middle of the channel is moving
fastest, so it is carrying the most sediment.
• Therefore, the most deposition occurs in the middle of the main channel, forming a
Midstream Bar and choking off the river and forcing it to split into Distributaries.
• Eventually, the
midstream bar may
grow so large it totally
blocks the flow of the
river.
• This causes Avulsion,
whereby the river will
get deeper and
eventually break the
Natural Levee at an
upstream location.
• Now the river will enter
the standing water at a
new location causing a
new delta to form.
The Migrating Mississippi Delta
• Avulsion has caused the Mississippi
river delta to migrate through time.
Engineers now help keep the river on
its present day track so that it still
flows through New Orleans.
Urbanization and Flash Floods
• Floods may be normal natural events, but urbanization causes fast
flooding events called flash floods, to be more likely.
• In a natural setting, rainwater can infiltrate into the ground and
vegetation will slow sheetwash.
• In highly urbanized settings, rainwater cannot infiltrate asphalt and
cement and there may be little or no vegetation to slow sheetwash.
• As a result, the increased discharge in rivers lasts a shorter time, but
increases faster and by a larger amount