LAB 5
RIVERS AND FLUVIAL LANDFORMS
OBJECTIVES
 To use air photos and topographic maps as complements to each other, gaining information
from photographs that is not shown on maps.
 To use air photos and topographic maps to identify fluvial landforms.
 To calculate the sinuosity (P) and gradient of a channel from a topographic map.
 To differentiate alluvial channel pattern types based on morphological features that can be
observed and measured on a map or air photo.
INTRODUCTION TO FLUVIAL GEOMORPHOLOGY
Stream-channel processes are responsible for shaping much of the land around us. Streams not only
erode and transport material from the land, but they also deposit sediment to create new landforms.
Because streams are such an important component in the formation of landscapes, there is an entire
sub-field devoted to their study, known as fluvial geomorphology.
Channel Types: bedrock and alluvial channels
Streams carry water and sediment from discrete areas known as drainage basins or watersheds. The
amount of water and sediment supplied by a basin depends on factors such as the amount of rainfall,
the erodibility of the land, and the size of the basin. These factors vary from place to place, and
accordingly, there are differences in processes and morphology from one stream to another. Some
rivers flow on or very close to bedrock. These bedrock channels are usually quite steep, flow in
narrow valleys, and have little opportunity to build fluvial landforms. Other rivers, termed alluvial
rivers, flow upon a relatively thick layer of fluvial sediment, or alluvium, and have greater ability to
modify their channel and create floodplains and other fluvial landforms, such as bluffs and terraces.
Figure 5.1. Can you differentiate bedrock channels from the alluvial river? (from A.H. Strahler and
A.N. Strahler, Modern Physical Geography 4th ed., J. Wiley and Sons Inc. ©1992)
Geography 103, Term 2
Lab Exercise 7 - 1
Geography 103, Term 2
Lab Exercise 5 - 2
Alluvial Channel Patterns: Straight, Meandering, and Braided
Alluvial stream channels exhibit patterns that can be classified as straight, meandering or braided. In
reality, there are no sharp or discrete distinctions between these channel types; rather, channels lie
along a continuum between two extremes. Nevertheless, a generalized classification can be made based
on whether a channel is composed of one or more sub-channels (single- or multi-thread) and the ratio
of channel length to valley distance, otherwise known as sinuosity (Figure 5.2). We will learn how
calculate sinuosity in the next section.
Straight channels are single-thread channels with a sinuosity < 1.5. Note that no natural channel is
every truly “straight,” thus they may also be termed irregular or non-meandering channels. In crosssection, straight channels usually have a deep pool along one bank a bar along the other (Figure 5.3a).
Meandering channels are single-thread channels with a sinuosity > 1.5. Typically, they carry a higher
proportion of their load as suspended material and tend to be narrow and deep with cohesive banks.
Meandering channels are similar in cross-section to straight channels (Figure 5.3b).
Braided channels have multiple channels, divided by islands or bars, that successively meet and redivide. Sinuosity is generally < 1.5. Typically, these streams carry a higher proportion of bedload and
tend to be wider, shallower and straighter, with more erodible banks. Braided channels are also termed
anastomosing (Figure 5.3c).
Figure 5.2. Classification of alluvial channel patterns based on number of channels and sinuosity.
Figure 5.3. Planform and cross-sectional views of a) straight, b) meandering, and c) braided channel
patterns.
Fluvial Landforms
The three alluvial channel pattern types are associated with different landforms, created by the erosion
and deposition of sediment by the river.
Landforms associated with all channel patterns in include floodplains and terraces. Floodplains are
formed during high flows when the river overflows its banks and deposits its sediment load. When the
sediment load of the river is low, however, excess flow may incise the floodplain, producing terraces
(Figure 5.4c)
Geography 103, Term 2
Lab Exercise 5 - 3
Figure 5.4. A braided channel (a) flows over alluvium. Lateral cutting by meander bends erodes the
alluvium (b), ultimately forming a series of alluvial terraces on either side of a relatively narrow flood
plain in the floor of the valley (c) (from A.H. Strahler and A.N. Strahler, Modern Physical Geography
4th ed., J. Wiley and Sons Inc. ©1992).
Meandering and straight channels will deposit sediment on the inside of a channel bend, forming a
point bar. Erosion occurs on the outside of a bend, forming and eroding (or undercut) bank and a pool
(Figure 5.5).
Figure 5.5. Features of straight and meandering channels (from J. Pitlick, Lab Manual for
Environmental Systems: Landforms and Soils 3rd ed., Burgess International Group, Inc.  1993)
Geography 103, Term 2
Lab Exercise 5 - 4
Overtime, a meandering channel will become increasingly sinuous (Figure 5.6) until the main channel
cuts through the meander neck. The cut off meander is then separated from the main channel by
deposition of sediment, forming a U-shaped body of water called an oxbow lake. Patterns of erosion
and deposition also cause the river to migrate across its floodplain. This can often lead to the
deposition of sediment in distinct ridges across a point bar, forming features known as meander
scrolls.
Figure 5.6. Planform and cross-section profiles of a meander bend of an alluvial river. The river is
flowing from the top to the bottom of the diagram. Small arrows show the swiftest current (from A.H.
Strahler and A.N. Strahler, Modern Physical Geography 4th ed., J. Wiley and Sons Inc. ©1992).
Gravel and sand deposits in braided channels produce mid-channel bars or islands. Bars visible at low
flow are often submerged at high flow.
Quantifying Channel Morphology: Calculating Sinuosity and Stream Gradient
Many features of channel morphology can be seen on topographic maps and aerial photographs,
especially when viewed in stereo. We can compare rivers in different settings by measuring some of
their characteristics on topographic maps. Flow depth or velocity cannot be assessed this way, but we
can measure the river’s width (w), gradient (or slope, S), and sinuosity (P).
Calculating Sinuosity
Sinuosity (P) is calculated as the ratio of the channel length (lc) to the distance of the valley (lv):
P = l c / lv
To measure channel length, use a piece of string to measure the length of the channel, being sure to
include all bends of the channel. Then use the string to measure the length of the valley, using the
contour lines on the map to make sure you stay within the valley. Calculate sinuosity by dividing
channel length by valley length measured on the map.
Calculating Stream Gradient
Stream gradients are generally much less than one degree so it is more convenient to express the slope
either as percent or as the change in stream channel elevation, measured in meters, per length of
channel, measured in kilometres. To express the gradient in units of “metres/kilometre”, calculate as
follows:
Geography 103, Term 2
Lab Exercise 5 - 5
Stream Gradient = Rise =. Vertical distance in m .
Run
Horizontal distance in km
Vertical distance is the change in elevation between a point upstream and a point downstream on the
river. This can be determined from the contour lines on the map.
Horizontal distance is measured as the length of the channel (including channel bends) between the upand downstream points. Distance is first measured on the map and then converted to distance on the
ground using the RF scale. See the steps below.
1.
Determine vertical distance:
Locate benchmarks or contour lines at upstream and downstream positions along the reach of the
stream.
Determine the elevation of each point and calculate the change in elevation as the upstream
elevation – the downstream elevation.
(Note: Maps may express elevation in metric (m) or imperial (ft). If units are in feet, convert your
vertical distance to meters using the conversion 1 ft = .3048 m)
e.g. A change in elevation of 100 ft = 30.48 m
2.
Determine horizontal distance:
Use a piece of string to measure the length of the channel on the map, being sure to include all
bends of the channel.
Convert to horizontal distance in km on the ground using the RF scale.
e.g. If RF is 1:50,000 and channel length (on the map) is 10 cm:
10 cm x 50,000
= 500,000 cm on the ground
= 5 km on the ground
3. Calculate stream gradient as the vertical distance (in m) divided by the horizontal distance (in km)
e. g. Stream gradient
Rise
Run
=
30.48 m
5 km
= 6.10 m/km
OR
=
30.48 m x 100 = .61%
5,000 m
AN INTRODUCTION TO AIR PHOTOS
In general, the reference information for an air photo is indicated on the photo itself. Additional
information is provided in air photo catalogue. Examples of catalogues are available in the GIC.
Source, Date, Roll and Photo Numbers
Photographs taken in BC after 1977 have a reference code indicated in the margin of the photo.
e.g. BC78081
The “BC” indicates that the photograph was taken by the Geographic Data Division of the BC Ministry
of Environment, Land and Parks in Victoria. The “78” indicates the photo was take in 1978. The last
three digits, “081” are the film roll number and relate to the flight line.
Geography 103, Term 2
Lab Exercise 5 - 6
The photograph number follows this reference code. Air photos take prior to 1977 do not have the BC
five-digit number. To determine when and where the photos were taken you will need to consult an air
photo catalogue.
Altitude, Focal Length, Time and Date
On most air photos, there are four dials on one side of the photo. Often the information is difficult to
read from the photographs – it is also included in the air photo catalogue.
1. The first dial is the altimeter, which shows the flying height of the aircraft above sea level.
2. The second dial is a bubble level that shows whether or not the plane was level at the time of the
photograph. The number “C 152.91” on the bubble is the focal length of the camera. The focal
length is 152.91 mm or 6”. The number Nr 124223 is the camera number.
3. The third dial is the clock showing the time the photo was taken (standard time).
4. The fourth dial gives the date the photo was taken.
Scale of Air Photos
In order to correctly identify photo images, it is important to know the scale of the photograph. Unlike
topographic maps, the scale is not usually written on the photo. Instead, information from the
instrument panel on the margin must be used to estimate the scale.
The scale of the photo is derived from the following equation: s = H/f
where f is the focal length of the lens and H is the average flying height above ground level (see
previous section), and s is the scale denominator, so 1:s is the scale presented as a representative
fraction (RF). In terrain with high relief scale will vary since, for example, valley bottoms will be
further from the camera than mountaintops. Calculations must be made for the elevation range of
interest.
e.g. calculate the scale for a photo taken at an altitude of 1600’ above a river valley. The
camera focal length is 6”.
s = H = 1600' = 1600' = 3200
f
6"
0.5'
This means that the scale for the river valley is 1:3200.
Air Photo Interpretation
The basis of air photo interpretation is the ability to identify the objects (usually by shape, size, and
grey tone) and infer the object's origin often by associating it with other adjacent forms. When
interpreting landforms, major features such as rivers, glaciers, mountain ridges and valley floors
usually stand out clearly. Observations may be made of landform development, such as evidence of
recent river channel changes, as revealed by abandoned channel segments; evidence of longer-term
valley or river base-level change, as shown by river terraces flanking the modern floodplain; and
evidence of recent hill slope instability noted by hummocky ground sloping up to clearly marked head
scarps (freshly broken rock material) or areas of lighter tone. In Canada many features related to
glaciation are usually visible. These include erosional features such as cirques, arêtes, horns, and icemoulded mouton forms, and a variety of depositional features (moraine ridges, eskers, till plains, etc.).
Vegetation and cultural features often obscure the details of landforms, so that landform interpretation
is easiest in alpine and Arctic areas, and in semi-arid and arid environments.
Geography 103, Term 2
Lab Exercise 5 - 7
LAB 5 - ASSIGNMENT
For the following questions, you will need the following topographic map and air photos.
Reminder: do not draw on maps or photos.
Map Reference: Surveys and Mapping Branch. 1975. Walker Creek 93H/15 2nd ed., 1: 50,000.
Department of Energy, Mines and Resources, Ottawa Canada.
Air Photo Reference: Geographic Data. 1978. BC78081: 76-84 [1:40,000]. Ministry of Environment
Lands and Parks, Victoria, BC.
PART 1: AIR PHOTOS AND TOPOGRAPHIC MAPS
1. Select air photos number 76 and 77. Compare the photos with the Walker Creek map to identify the
north edge of the photos. You may need to look at the original air photos to answer questions 1c and
1d.
a. In what year was photo number 76 taken?
_____________
b. What is the roll number?
_____________
c. What was the altitude of the plane when the photo was taken?
_____________
d. Was the plane level when the photo was taken?
_____________
e. Is this reference information on the N, S, W or E edge of the photograph?
_____________
2. On the air photos, try looking for areas of different terrain, different vegetation types, and different
river types. Note human features: can you tell the difference between forest harvesting areas and
alpine areas? Can you tell roads from streams? (No need to write any answer here.)
3. Name the river in the photographs.
4. In which direction does the river flow? Briefly explain your answer.
5. Locate the island in the river, half of which can be seen the southeast corner of photo number 76.
Locate it on the topography map. Give the 6-digit UTM coordinates for the centre of the island.
Geography 103, Term 2
Lab Exercise 5 - 8
PART II: RIVER CHANNELS AND FLUVIAL LANDFORMS
1. Describe the alluvial channel pattern of Kitchi Creek in a single word! Look at the legend on the
back of a newer topographic map to determine what the brown dots adjacent to the river on the
represent. Now examine Kitchi creek on air photo BC78081: 83. How does the creek on the
photograph differ from the creek depicted by the map?
2. Consider the McGregor River in the northwest section of the map. What map evidence is there that
meanders have shifted in the past? What further evidence can you see on the air photos?
3. Using both the photos and the map, identify the landform at each of the 10 locations listed below.
Select from the following list of 8 landform types. Some types will be used more than once.
Flood plain
Meander scroll
UTM Location
402794
402793
427816
343814
496845
375785
344815
340822
448827
355810
Point bar
Terrace
Landform type:
point bar
eroding bank
alluvial fan
point bar
mid-channel bar
flood plain
eroding bank
meander scroll
Terrace
Oxbow
Eroding bank
Alluvial fan
Oxbow lake
Mid-channel bar
Geography 103, Term 2
Lab Exercise 5 - 9
Compare the sinuosity, gradient, and channel pattern of two sections of McGregor River.
Section A:
from 475843 to 530836
Section B:
from 450835 to 311850
a. For each section, calculate sinuosity (P). Show your calculations.
b. For each reach, calculate gradient (S). Note that on this map the contour interval is in imperial units.
Show your calculations.
c. Examine air photo BC78081: 81 and refer to answers from 4(a).
Is Section A straight, meandering or braided? Explain your answer.
Is Section B straight, meandering or braided? Explain your answer.