STREAMS Chapter 12
A. Introduction
1. Surface
and near surface “storage” of water on earth
a.
Oceans hold about 97.2% of all surface water
b.
Glaciers hold about 2.1%
c.
Ground Water accounts for about 0.6%
d.
Streams & Lakes account for about 0.01%
e.
Atmosphere accounts for about 0.001% at any
given time
Although
the atmosphere and streams hold relative small amounts of water at any given
time, water in both is in motion, moving very quickly through the cycle, making
it very significant
2.
Hydrologic (Water) Cycle (see fig 12.3 p. 274)
driven by solar energy and gravity
Know
processes: evaporation, transpiration,
precipitation, infiltration, surface runoff, percolation
3. Definitions
a. STREAM---a
body of water flowing downslope along a definite path (called a channel)
In this sense, “stream” covers
rivers, creeks, brooks, branches, ditches, etc.
b.
Channel flow
vs. sheet flow---channel flow is predominant see
p. 272
c. Drainage
Basin---the geographic area from which surface runoff may reach a specific stream is the drainage basin (= watershed) for
that stream. Page 283-285, see Fig. 12.15, p. 285
d. (Drainage) Divide the boundary between
drainage basins
4.
Lakes: In geological terms, lakes are temporary
features that exist only because the drainage system has not yet had enough
development time to reach equilibrium and drain the lake.
5.
Climate effect
on streams
a.
In dry (arid)
desert environments only major streams flow year round. Water in the stream bed infiltrates to become
ground water
b.
In wet (humid)
environments the bottom of the stream channel normally intersects the ground
water table, hence there is base flow of GW to the stream----because of this, these streams rarely go completely “dry” even in fairly
severe droughts.
6.
Energy of a
stream (energy = the ability to do work)
a.
A stream
possesses potential energy by virtue of being on a land surface above
sea level. As the stream winds it way
toward the ocean, that energy is expended as kinetic energy
b.
Rain falling
reaching a stream at high elevations has much more potential energy than rain
falling on a coastal plane near sea level
c.
Most stream energy is expended through
friction between the water and i.) itself, ii.) the air interface, and iii) especially against the sides and
bottom of its channel. However, there is
still enough energy remaining for erosion and transportation of sediment to
make streams the most important process in sculpting landscapes.
1. Dendritic Pattern---most common; forms when all underlying material is equally susceptible to erosion
(Dendritic = “Tree-Like” pattern)
2. Trellis
Pattern---tributaries enter main stream a right angles. Typical of Valley and
3.
Radial
Pattern---all streams originate near a central point and flow (radially) away from that point
4.
Other
Patterns---ignore---not on test
1.
Discharge—depends
on average depth and width of channel filled with water (cross-sectional area)
and the stream velocity p. 272-273
2.
Velocity---depends
to some extent on amount friction with the channel, but most importantly is the
gradient----i.e. The “steepness” of the downhill slope of the channel p. 272-273 Velocity profile: see Fig. 12.5,
p.275
3.
Gradient---the
drop in elevation (in feet—or meters) of the channel over horizontal distance
(in miles--- or kilometers) down stream.
Stream gradients are usually higher nearer the headwaters and lower
nearer the mouth, giving a x-sectional plot of the
gradient that appears as a concave curve that “flattens out” near the mouth of
the stream. p. 272-273, see Fig 12.4, p. 274
4.
Base
Level----The depth to which a stream will erode it channel. Most streams erode laterally, not downward,
indicating that they have already achieved their (temporary) base level. Ultimate Base Level for most streams is mean
sea level. p. 286-287
1.
Extent of
erosion depends on the discharge and velocity of the stream. During severe flood stage, discharge can
increase by 50-100 fold, so much (most) erosion occurs during those brief times
the stream is at flood stage.
2.
Most streams
(in the eastern US) have achieved temporary base-level, so most erosion is
lateral, not downward
3.
Processes
a.
Hydraulic
action---force of water erodes---most important
b.
Abrasion---sediment
carried by stream rubs (abrades) channel
Leads
to rounded gravel and sand particles as well as scoured channel and potholes
1.
Streams are the
most important of all surface processes acting as transportational agents
2.
Definitions:
a.
Capacity---measure
of the total solid sediment a stream can transport—depends on discharge
b.
Competency—measure
of largest particle size a stream can transport—depends on the discharge,
particularly the velocity component.
c.
Load---solid
and dissolved material transported by a stream
3.
Types of
Material p.
276, cover more in class than in the book
a.
Bed Load—usually
larger sized particles (sand and gravel). Most of the time material is not
moving continually; get movement by traction (dragging along the bottom) or
saltation (bouncing along the bottom.
b.
Suspended
Load---usually (except for flood stages or streams with high discharge) clay
and silt sized particles. Sand and
gravel are typically moved during flood stages.
Very efficient as solid sediment moves at the same speed as the water.
c.
Dissolved
Load---dissolved mineral matter carried in solution. Streams are considered as “fresh water”,
which indicates that the amount of dissolved mineral matter is not that great.
2. Types of stream deposits
a. (Sand) Bars—most (point) bars form on the inside bend of a
stream (low energy side) p. 279—see Fig. 12.9.a
and 12.9.b
b. Flood
Plain---deposits (including Natural Levees) p278
c. Alluvial
Fans---see p. 280 and Fig. 12.14, p. 283
d. Deltas— see p. 280 and
Fig. 12.13, p. 282
Note: This material is covered in much more detail
in class than in the textbook—so take good notes
1. With
time as stream (and mass wasting) erode a landscape there is a definite progression
followed by both the landscape itself and the streams that drain (and erode)
it.
2. William
Morris Davis and the concept of
different stages of development of landscapes and their streams as going
from “youthful” to “mature” to ”old age”, with each stage having its own
characteristics although these stages grade continually, gradationally, and
imperceptibly from one to the other.
Absolute years cannot be placed on durations of the stages as some
streams and landscapes develop (or “age”) more rapidly than others.
3. Characteristics
of each stage:
a. Youthful
Stage
i.
Characterized by waterfalls and rapids
ii.
Deep V-shaped valleys and canyons
iii.
Headward
erosion of tributaries
iv.
High relief topography; mass wasting mostly rockfalls, slides.
v.
Erosion of channel is downward
b. Mature
Stage
i.
Streams develop floodplains
ii.
Progresses to extreme meanders and oxbow lakes
iii.
Rolling topography; mass wasting mostly slump,
creep
iv.
Erosion of channel is lateral, not downward
c. Old
Age Stage---hypothetical end stage; not reached in nature
i.
Flat, featureless land surface called a peneplain
ii.
Extensive floodplains, wide shallow channels
4. Rejuvenation
a. Incised (= Entrenched) Meanders p. 290-291,
see Fig. 12.23, p. 291
b. River Terrace Deposits p. 290, see Fig. 12.22, p. 291