Stream/Groundwater
I.
Hydrologic Cycle and Streams
A. Hydrologic Cycle – Water is evaporated from the
seas, carried inland and is precipitated as rain or
snow. More than half of the water is evaporated
rapidly. The remainder either flows over the
lands surface as runoff in streams or percolates
down into the ground to become ground water.
B. Stream - Is a body of flowing water contained
within a channel, regardless of its size.
1. headwaters = upper part of stream near its
source
2. mouth = place where a stream enters sea,
lake, or larger stream
3. stream channel = long narrow depression
eroded by the stream into rock or sediment
4. bed = bottom of channel
5. banks = sides of channel
C. Drainage Basins – The total area drained by a
stream and its tributaries (a tributary = is a small
stream flowing into a larger one).
1. A ridge or strip of high ground dividing one
drainage basin from another is termed a
divide.
a. ex. Continental Divide in U.S.
2. Drainage Pattern = Is a map view of a river
and its tributaries. Pattern is determined by
the slope of the land and underlying rocks.
3. Types of drainage patterns
a. dentritic = resembles branches of a tree
(tributaries join stream in a variety of
angles).This is the most common type.
II.
b. radical pattern = carries water away
from the center of a high mountain or
volcano in all directions (like spokes of
a wheel)
c. rectangular pattern = in which
tributaries have frequent 90 degree
bends and tend to join other streams at
right angles. This pattern develops
when there is a network of fractures
which erode more easily than unbroken
rocks.
d. Trellis pattern = consists of parallel
main streams with short tributaries
meeting them at right angles.
Factors Affecting Stream Erosion and Deposition
A. Velocity is the most important factor controlling
erosion and deposition. Velocity = distance water
travels in a stream per unit time.
1. maximum velocity is near the middle of the
channel.
2. high velocity (meaning greater energy)
generally results in erosion and deposition.
3. low velocity causes sediment deposition
4. factors that control a streams velocity
a. gradient = the downhill slope of the bed.
1. measured in feet per mile
2. usually the gradient decreases
downstream. ( steeper=faster)
b. channel shape
1. larger channel has more surface
for moving water to drag against
(more friction)
2. shape can be narrowed by human
interference (bridges) which
increases velocity
c. roughness of channel also controls
velocity. A stream flows more rapidly
over a smooth channel (less friction)
d. discharge the volume of water that
flows past a given point in a unit time
1. measured in cubic feet per second
2. discharge increases down stream
in humid climates
i. to handle this increase in
discharge stream increases in
width and depth downstream
(some increase in velocity)
III.
Stream Erosion – Streams erode rock in three ways
A. Hydraulic action = ability of water to pick up and
move rock and sediment
B. Solution = rocks dissolved by water (stream
flowing over limestone may dissolve the rock)
C. Abrasion = the grinding away of the stream
channel by the friction and impact of the sediment
load
IV. Stream Transportation of Sediment
The sediment load transported by a stream can be
subdivided into bed load, suspended load, and dissolved
load
A. Bed load = large or heavy sediments particles
that travel on the stream bed
1. Particles move by
a. Traction = rolling, sliding dragging
b. Saltation = short leaps or bounces
B. Suspended load = sediment that is light enough
to remain lifted indefinitely above the bottom by
turbulence.
C. Dissolved load = soluble products of chemical
weathering.
V. Stream Deposition
Whenever a stream slows down sediments begin to drop
out. Along a stream course there are cycles of deposition
and erosion, forming bars and flood plain deposits. At the
end of a stream sediments may be deposited in deltas or
alluvial fans. Deposition occurs because there is a
decrease in a streams competence and capacity
1. Causes for decrease
a. Loss of velocity in stream
b. Obstacles
c. Overloading in stream
d. Widening of stream bed
e. Reduction in gradient
A. Bar = a ridge of sediment, usually sand or gravel,
deposited in the middle or along the banks of a stream.
Placer deposits = running water has mechanically
concentrated heavy sediment.
B. Flood Plains
1. Flood plain = a broad strip of land built up
by sedimentation on either side of stream
channel
a. when flooding, stream spreads over
plain and reduced velocity causes
deposition
b. natural levees- floods deposit coarse
sediment in strips bordering the
channel
VI.
c. poorly drained area behind levee called
backswamp
C. Meandering Streams and Point Bars
1. Meanders = streams that flow on flood
plains move in sweeping bends. Outside
bend has highest velocity and has most
erosion. Inside bed the low velocity causes
deposition in the form of point bars.
2. When a meander gets cut off it becomes a
crescent shaped oxbow lake.
D. Deltas
1. A stream flowing into standing water loses
velocity and usually builds a delta.
2. Shape of delta depends on the balance
between sediment supply from the stream
and power of waves and tides.
E. Alluvial fan = large fan or cone shaped pile of
sediment that forms where a streams velocity
decreases as it emerges from a narrow mountain
canyon onto a flat plain.
Stream Valley Development
A. Down cutting and Base level
1. Downcutting = the process of depening a
valley by erosion of the stream bed.
Vertical erosion.
2. Streams cannot cut below sea level, or it
would have to flow uphill to get to the sea.
This limit of downcutting is called base
level.
B. Lateral erosion = when the valley widens by
erosion when a stream meanders.