Sampling

If you want to study an ecosystem such as a sand dune, lithosere, hydrosere or a saltmarsh, you probably won’t be able to study the entire area due to time / size constraints (or the high probability of complete boredom setting in). Therefore you will need to sample from the ecosystem in order to collect data that is accurate and representative of the area as a whole. There are many different types of sampling; here we shall consider methods of random and systematic sampling as these are the two of most practical use for geographical fieldwork in these environments.

Random sampling

is used to study a homogenous area; when the study area is the same throughout (for example a flat meadow). As it is reasonable to assume that the environmental conditions do not change within the meadow, it doesn’t matter whereabouts within the area you take your samples from. However one thing of vital importance is that you do not choose sample sites yourself, as this will introduce bias. Random sampling is achieved by generating two random numbers (from a random number table or a scientific calculator) and using them as co-ordinates for the placement of a quadrat. This is illustrated below:

Systematic sampling

is used when the sampling area includes an environmental gradient; where physical conditions change within the locality (for

example a sloped hillside where factors like wind speed may increase as you move up the hill). Sampling of hydroseres, saltmarshes and sand dunes requires this approach, as the environmental gradient in these habitats is very distinct. Systematic sampling involves having structure to your method in order to obtain data in a series, rather than at random.

A transect is required to systematically sample through an environmental gradient. Transects can be of many types, the two most common being line transects (sampling along a line, e.g. a series of points along a tape measure) and belt transects(sampling within an area, e.g. a series of quadrats).

line transect belt transect

Belt transects can be of two main distinctions. Continuous belt transects sample in an unbroken manner through the entire environmental gradient (for example by turning a quadrat end-over-end). Interrupted belt transects leave some areas un-sampled (e.g. placing a quadrat at 10 metre intervals). Of these two types of transects, the interrupted belt transect is the one most frequently used for conducting fieldwork within large areas e.g. saltmarshes or sand dunes.

continuous belt transect

interrupted belt transect

Quadrats

When carrying out any form of vegetation sampling, one piece of equipment you will almost certainly require is a quadrat. Quadrats are defined simply as sampling areas, and can therefore be of almost any shape, size and type.

Frame quadrats are 2-dimensional empty frames of any known area. These days they are usually square as this makes for easy calculation of the sample area; however they can be any shape – the Victorians favoured circular ones. Frame quadrats allow you to obtain data as direct counts (exactly how many of each species there are inside the quadrat); or as percentage cover (an estimate of how much of the quadrat area is taken up by each species).

Grid quadrats are also 2-dimensional quadrats, this time divided into a known number of small squares (often 100). By doing this it is possible to generate percentage frequency data (how many times a species occurs; present in 1 square = 1% frequent), as well as direct counts and more accurate percentage cover data (if a species fills 4 squares the coverage is therefore 4% - there is less estimation involved).

Point quadrats are 3-dimensional quadrats and therefore ideal for sampling vegetation, which tends to grow in layers or canopies. A point quadrat consists of a frame with 10 holes which is inserted into the ground by a leg. A pin is then dropped through each of the holes in turn, and the species that the pin touches are recorded. In this way the total number of pins touching each species can be converted to percentage frequency data (if a species touched 6 out of the 10 pins it is 60% frequent).

Any style of quadrat can be used to sample ecosystems such as saltmarshes or sand dunes; the final decision will depend on a number of factors including availability, cost, time, practicality and ease of use, as well as the type of data required. There are both advantages and disadvantages to all quadrats (grid quadrats are robust and easy to use, yet can be time-consuming and destructive to vegetation; point quadrats are generally quicker and less destructive, however rare or small species can be missed). Ultimately the type of quadrat you choose to collect your data does not matter, provided you can justify why you chose it.

Measuring abiotic factors

Environmental conditions are important to consider when conducting fieldwork, as changes in vegetation are often as a response to changes in the physical and chemical environment. N on-living environmental factors are known as abiotic factors, and can include light, temperature, water, atmospheric gases, wind, humidity, and soil conditions to name but a few. Abiotic factors cannot be controlled, however can (and should) be monitored. Methods for measuring abiotic factors vary greatly according to the time and equipment available. Some of the more common methods are given below:

Temperature can be recorded using a simple thermometer or more precisely with digital probes or data-loggers if they are available. Wind direction can be determined with a compass, and wind speed recorded with an anemometer. Humidity can be monitored with the use of a whirling hygrometer or digital probes. Soil characteristics can be analysed in all manner of ways; soil moisture can be recorded with soil probes in the field, or can be determined from the analysis of soil samples in an oven, as can soil organic and inorganic matter. Simple chemical tests can determine nitrogen, oxygen, phosporous, and sodium content; and pH can be determined with soil probes, indicator solution or a Litmus test. The comapction of the soil substrate can recorded with a soil penetrometer if required.

Regardless of which abiotic factors are to be measured, or indeed the technique employed to do so, there are two imprtant rules when measuring abiotics. The first is that repeat readings must be

taken (one sample is not enough), and the second that all samples must be taken during the same time period (as close together as can realistically be achieved).

RIVER SURVEY

Background information

Models of downstream change

(i) Bradshaw model

(ii) Schumm model

Discharge

Discharge is the volume of flow of water per unit time, often measured in cubic metres per second (or cumecs).

In general, discharge increases with distance from the source to the mouth of a river. The extra water enters the river from tributary streams, surface runoff, throughflow and baseflow.

Anomalies

Sometimes there is not a steady increase in discharge with distance from the source.

Human influences - Humans can either increase the discharge (e.g. at a sewage outfall) or decrease the discharge (e.g. abstraction of drinking water).

Land use - discharge is higher in unvegetated, urbanised and deforested basins because there is greater surface runoff.

Rock type and structure - the surface runoff component of discharge is lower in drainage basins of permeable rock than impermeable rock.

Tributaries - the precise increase in discharge when a tributary meets the main river depends on the size of the tributary. Clearly a small first-order stream will add a lower discharge component than a much larger tributary

Velocity

The velocity of a river refers to the rate of water movement, often measured on metres per second. Mean flow velocity increases slightly with distance from the source. Although velocity appears to be higher in mountain streams (where there is a lot of white water) than in the lowlands, appearances are deceptive! Much of the water in the upper course stream, particularly close to the bed and banks, is almost stationary.

River velocity is determined by the efficiency of the river in overcoming friction with the bed and banks. Approximately 95% of a river's energy is lost to friction. Velocity increases as a river becomes more efficient in its lower course. Why?

Shape of the channel - the river is deeper, wider and has higher discharge in its lower course. Relatively less water is in contact with the wetted perimeter, so friction from the bed and the banks is reduced.

Channel roughness - pebbles, stones and boulders on the beds and banks increase the roughness of the channel. The wetted perimeter is higher, increasing friction and reducing the friction of the river. Channel roughness is higher in the upper course than further downstream.

Hydraulic radius

Hydraulic radius = cross sectional area / wetted perimeter.

Hydraulic radius is a measure of the efficiency of the the river channel. The higher the hydraulic radius, the more efficient the river channel is.

The more efficient the river is, the more energy the water will have to

move downstream (so as hydraulic radius increases, velocity increases)

carry load (so as hydraulic radius increases, the river's competence and capacity increases)

increase the rate of erosion (in the upper course, as hydraulic radius increases, there is a higher rate of vertical erosion, so gradient increases; further downstream where the river is closer to base level, as hydraulic radius increases, there is a higher rate of lateral erosion).

Gradient

The gradient of the river bed is the ratio between vertical fall over horizontal distance. It is drawn as a graph of distance from source (x-axis) against height above sea level (y-axis). Bed gradient decreases gradually with distance from the source, shown as a downward concave curve.

Changes in gradient are related to changes in discharge. Discharge is higher in the lower course. Since gradient decreases as discharge increases, a river can transport the same quantity and size of sediment load in the gentler lower course as it can in the steeper upper course.

Anomalies

In practice, the long profile is not always a smooth downward curve.

Pools and riffles - on a small scale, particularly in the upper course, the long profile is stepped. The gradient is steeper at riffles but gentler at pools. Over the whole course of a larger river from source to mouth the pool-riffle sequence is too small to show up on the long profile, but the features may show up on a graph of the gradient over a few hundred metres of the river's course.

Tributaries - downstream of the confluence of tributaries, the gradient may increase because discharge (and therefore the rate of downward erosion on the river's bed) is higher than upstream of the confluence.

Rock type and structure - where a river flows from a band of more resistant rock to an area of less resistant rock, a waterfall may form. In terms of the long profile graph, the waterfall is a sudden increase in bed gradient.

Knickpoints - these occur as a result of a fall in base level, also known as rejuvenation. Sea-level may fall in relation to the land or the land may rise in relation to the sea. A waterfall is formed which will over time retreat upstream as it is eroded.

Load - a sudden increase can lead to increased vertical erosion. Human activity can cause this - for example, afforestation of the drainage basin reduces sediment load in the river.

Load

Load is the total mass of material transported by a river. The way in which material is moved depends on its size. There are downstream changes in the amount and the mean particle size of load.

Type of loadType of

particlesDiameter of

particlesHow they are transported

Bed load Sand, pebbles Over 0.1mmSaltation and traction

Suspended load

Clay and silt 0.001 - 0.1 mm Suspension

Dissolved load

Soluble material

- Solution

The competence of flowing water is the maximum size of particle that the river can transport. The capacity of a river is not the amount of water it contains (this is the discharge) but the maximum amount of load that the river can transport.

Therefore, a river's competence increases as the water velocity increases. At low velocity only lay and silt can be transported. As the velocity increases, larger particles such as sand and pebbles, can be transported. However, the relationship between velocity and the size of particles transported is not a simple positive correlation.

The mean particle size decreases with distance downstream. This is not because the competence of the river has decreased. Instead smaller particles have become a relatively more important component of the load. Why?

More time for erosion - the major source of pebbles and stones in the river is from the river's upper course. Rocks fall into the channel in the upper course and are eroded from the beds and banks. The further these rocks are carried downstream, the more time there will have been for them to have been eroded by attrition and abrasion. Abrasion reduces the size of rocks, while attrition makes rocks both smaller and rounder.

More time for weathering - much of the river's erosion occurs at times of high discharge. During times of low flow, stones are stored on the beds or banks. The longer the stones spend in storage, the longer they will be affected by weathering processes (such as frost shattering).

Sorting - the river sorts particles of different sizes. Smaller particles are carried at lower velocities. These particles remain in the water flow during periods of low flow when larger particles are deposited.

Anomalies

There are two reasons why a downstream decrease in particles does not always occur.

Influence of tributaries - stones and pebbles entering from a short tributary may be larger and more angular than those in the main river.

Human activity - stones and pebbles can be added by people, e.g. mining spoil can be large and angular

Stream ordering

Many geographers have tried to classify the different streams in a drainage basin. Stream ordering can be useful for fieldwork if you want to compare sections of different streams in the same drainage basin. The most popular way of doing this is Strahler's method. A stream segment with no tributaries is designated a first-order segment. A second-order segment is formed by the joining of two first-order segments, a third-order segment by the joining of two second-order segments, and so on.

There is no increase in order when a segments of one order is joined by one of another order (such as a first-order and a second-order stream). Some researchers have suggested that as order increases by one, discharge is approximately double.

Questions to investigate

There are many possible questions that you could investigate

How and why does velocity / discharge / hydraulic radius change downstream in river x?

To what extent does river x follow the Bradshaw model? Does stream order affect hydraulic radius and velocity in

drainage basin x?

Choosing a fieldwork location

Before you start the investigation in full you must visit your chosen river and carry out a full pilot study. In a pilot study you check that the river is safe for fieldwork and that you can get access where you need to sample.

Investigations into channel variables are best carried out in the upper course of streams and rivers, within a few kilometres of their source. As you need to be able to measure the width and depth of the channel, make sure that water in the stream comes to no higher than you knees at the deepest point. Fast-moving water deeper than this can knock you over. It is possible to drown in 10cm of water.

If you are focussing on one stream (e.g. measuring a range of river variables with distance downstream), choose at least 3km of length. If you are carrying out work on several streams in a drainage basin (e.g. comparing first-order and second-order streams), choose at least 2 square km of the drainage basin.

Stage 2: Sampling within a drainage basin | Measuring river variables

Sampling within a drainage basin

Even if you are just focussing on a 3km stretch of a stream, it is not necessary to measure every part of the river. Instead you need to design a sampling strategy that will yield representative data without bias.

For investigating downstream changes in one river, the most straightforward way to do this is to take a systematic sample. Select 10-15 sample points at regular intervals (e.g. a 50m distance between each site). Your choice for the distance between sample points will need to be justified; make sure that they are representative of the whole length of the river. You may also wish to make sure that there are sample points just upstream and just downstream of major features of interest, such as a point where tributaries join or where water is abstracted. If you are comparing the characteristics of more than one stream, make sure that you that use the same sampling strategy for each.

Measuring river variables

Depth, width and wetted perimeter | Velocity and discharge | Long profile | Bedload | Suspended load

(i) Depth, width and wetted perimeter

At each sample site, measure

(a) Channel width - use a tape measure or rope held across the channel from bank to bank on the water surface.

(b) Channel depth (or wet depth) - take a number of readings across the channel (e.g. ¼ across,1/2 across, ¾ across) to measure the depth from the water surface to the stream bed.

(c) Wetted perimeter - use a tape measure or chain to measure the total distance that water is in contact with the bed and the banks. You may need to secure the tape in position with stones or pegs, especially if the water is fast-moving.

The channel width, channel depth and wetted perimeter vary from day to day. If you are are sampling on one day only this is not a problem, but you can obtain additional information by measuring the bankfull width, depth and wetted perimeter as well. Bankfull is the stage where the river is completely filling its channel.

At each sample site also measure

(d) Bankfull width - use a tape measure or rope held across the channel from bank to bank. The edge of the channel may be marked by a change of slope and/or vegetation. Keep this in position to help you with (e).

(e) Dry depth - measure the depth from the bankfull height to the water surface. If you take readings in the same place as you measured channel depth (or wet depth), you can add the two figures together to find bankfull depth.

(f) Bankfull wetted perimeter - use a tape measure or chain to measure the distance from bankfull point on one bank to bankfull point on the other.

(ii) Velocity and discharge

The cheapest way that you can measure velocity and discharge is the float method. You need a 10m tape measure, a float (i.e. something that floats and is brightly coloured, such as an orange) and a stopwatch. You also need at least two people for this method.

Measure a set distance of the river and mark the start and end points. 10 metres is a good idea - it's long enough to show interesting variation in results, and a round number also makes the maths much easier! Put the float in the water slightly upstream of the start point. Using the stopwatch, time how long it takes the float to move from the start point to the end point. Repeat this procedure at least five times, placing the float at regular intervals across the stream so that you measure velocity across the channel.

It's pretty likely that at some stage the float will get stuck in an eddy! Either nudge the float to move it along or abandon that reading and start again.

The raw data that you have collected by this technique can be used to measure velocity and discharge. See Data Presentation.

(iii) Long profile

Although you can plot long profiles using Ordnance Survey map data, for more detailed information you should collect data in the field. You need a 10m tape measure, two ranging poles and a clinometer or Abney level. If you don't have this equipment, it is possible to produce a home-made alternative - see instructions below. You also need at least two people for this method.

To avoid standing in the river, you can use the gradient of the dry land on the banks as a substitute for the water surface slope. At each sample point on the river's course, measure slope angle from the sample site upstream and slope angle from the sample site downstream. See the diagram for how to do this.

If you don't have a clinometer, cut out a semi-circular piece of card. Using a protractor, calibrate the curved edge with angles. Fix a length of string to the centre of the straight edge of the card. Add a weight (such as a ball of blu-tak) to the other end of the string. Two people stand a measured distance along the gradient. The person at the bottom of the slope should align her eye with corner of the card, then read off the angle of the slope by lifting the card up so that its other corner is aligned with the eyes of the person upslope.

(iv) Bedload

A river's load can be classified into three groups: bedload (sand, pebbles and stones), suspended load (clay and silt) and dissolved load (soluble material). Although all types of load can be measured in the field, in practice measuring the bedload is the most stratightforward of the three.

Since both the volume of load (measured in cubic metres) and the competence of the river (the largest sized particle that the river can transport) increase exponentially when the river is in times of flood, samples of supended load made at times of low flow will not be representative of the whole year's load.

It is not, of course, possible to measure every stone in the river. Instead you will need to design a sampling strategy to give you a representative sample without bias that you can repeat at each sampling point. For example, you could collect 20 pebbles at random from the river bed at three different points across the width of the channel.

The simplest way to measure pebble shape is to classify the stone as very angular, angular, sub-angular, sub-rounded, rounded or very rounded. Decide which shape is the best fit for each pebble.

very angul

ar

angular

sub-angul

ar

sub-round

ed

rounded

very round

ed

For an estimate of pebble size, measure the longest axis of each pebble.

For more precise shape data, use Cailleux's Flatness Index to measure the degree of roundness. The raw data needed for each pebble is as follows.

1. The length of the longest axis (called l)2. The radius of the sharpest angle (called r)

To calculate the Cailleux Index see Stage 4.

For more precise size data, measure the a, b and c axes of each pebble. For pebbles where it is difficult to pick out the axes, allow the pebble to rest on a flat surface. The length of the longest axis is the a axis.

(v) Suspended load

A river's load can be classified into three groups: bedload (sand, pebbles and stones), suspended load (clay and silt) and dissolved load (soluble material). Although all types of load can be measured in the field, in practice measuring the bedload is the most stratightforward of the three.

Since both the volume of load (measured in cubic metres) and the competence of the river (the largest sized particle that the river can transport) increase exponentially when the river is in times of flood, samples of supended load made at times of low flow will not be representative of the whole year's load.

Calculations

(i) Calculating cross-sectional area

Cross-sectional area = channel depth x channel width

If you have taken more than one measurement of channel depth you can calculate the mean.

Alternatively you can plot the width and depth readings on graph paper, then count the area of the stream. Maths types may wish to use the trapezium rule to calculate the area.

(ii) Calculating hydraulic radius

Hydraulic radius is calculated simply as follows

cross sectional area ÷ wetted perimeter

If you have measured bankfull width, depth and wetted perimeter, you can also calculate bankfull cross-sectional area and bankfull hydraulic radius, and compare these to the figures calculated above.

(iii) Calculating velocity

Find the mean time for the float to travel the set distance at each site. Velocity is then simply calculated as

velocity = distance ÷ time

For example, if a float travels 10m in 8 seconds,

velocity = 10 ÷ 8

velocity = 1.2 metres per second

Of course, even if you started the float off at different points across the width, it is likely to have been sucked into the line of fastest flow, probably in the centre of the stream. The results are, therefore, biased towards the fastest parts of the stream, and will not fairly indicate the velocity elsewhere. To overcome this bias, multiply the result that you have calculated by 0.8. Studies have shown that this produces a more realistic indicator of velocity across the whole width of the stream.

So for the example above where simple calculated velocity = 1.2 metres per second

1.2 x 0.8 = 0.96 metres per second

(iv) Calculating discharge

To work out discharge for any particular point you first need to have calculated mean velocity and cross sectional area.

Discharge = velocity x cross sectional area

If velocity = 0.96 m/s and cross-sectional area = 1.25 sq m

discharge = 0.96 x 1.25 = 1.2 cubic metres per second (or cumecs)

If mass = 1.2g and velocity = 0.96 m/s

Kinetic energy = 1.148 joules per second

Data presentation

Tables of results

Produce a summary table showing the results (and averages) for each sample site.

Plotting river variables on scattergraphs

The most straightforward scattergraph that you can plot is distance downstream on the x-axis against one river variable (width, wetted

perimeter, cross-sectional area, hydraulic radius, mean depth, mean velocity, discharge) on the y-axis. Add a line of best-fit.

You can test the significance of the relationship between the two variables by calculating the Spearman's Rank Correlation Coefficient.

(iii) Displaying bedload data

If you have used Power's Index of pebble shape, you can display the data in proportional bar charts (see right) or pie charts to show downstream changes.

Data on bedload size (including the Cailleux Index) can be plotted on scattergraphs.

Statistical tests

You need to have collected data from at least 10 stations along the river. Calculate the Spearman's Rank Correlation Coefficient to test the significance of the relationship between distance from source and the variable (e.g. velocity, discharge, hydraulic radius, etc). Need more information about this test?

Stage 5: ReviewThe first stage of your conclusions is to describe the trends that you have found. Go through in turn each of your scattergraphs and Spearman's Rank results. For each

what is the direction of the relationship between the two variables? (e.g. positive correlation / negative correlation / no clear relationship)

what is the quality of the relationship between the two variables? (e.g. statistically significant / weak / no clear relationship)

are there any anomalous results?

Now try to give reasons for the direction and strength of the relationship between the variables. Also try to give reasons for any anomalies. Here are some ideas to get you started

What is the effect of tributaries on the main river? Is there a step change in some of the variables?

What is the effect of human activity? Is water abstracted or water added?

What is the effect of rock type? Does the river flow from more resistant to lesss resistant rock?

In your evaluation section, discuss the reliability of your data collection techniques. To what extent do you think that your results are accurate? What other data would it have been useful to obtain?

Why do rivers meander?

Meanders are the sinuous bends in a river, possibly named after the River Menderes in south-west Turkey.

It is thought that turbulent flow, rather than straight flow (also known as laminar flow) is a fundamental characteristic of the movement of fluids.

Simulations with flumes (laboratory models of rivers) have shown that, above a critical minimum velocity, meanders will form. Friction with the channel bed and banks causes turbulence in the water flow, which promotes the development of alternating bars of sediment along the channel.

A helical flow (corkscrew shaped flow) is established, with the water surface being elevated on the outer bank of each curve, and return currents at depth directing the flow towards the opposite bank. The outer bank is eroded as a result of the higher flow velocity, whereas deposition takes place on the inner bank, and forms a point bar.

Characteristics of meanders

Deposition occurs on the inside of the meander curve where depth of water is lowest. As much of the water is in contact with the bank, friction is high, so water velocity is low. The moving water deposits all but the smallest particles that it is transporting. The resulting deposit is called a point bar.

In contrast erosion (especially hydraulic action of the bank) occurs on the outside of the meander curve. Less water is in contact with the bank, friction is lower, so water velocity is higher. Erosion creates a steep river cliff.

Choosing a fieldwork location

If you are investigating a short stretch of river or a single meander, take at least ten sample points. You should only carry out this investigation if you can safely reach the bed of the river across its width. The key data that you need to collect are channel width/depth and velocity at different depths.

Displaying your data

Velocity

If you have used a flow vane and have collected enough readings from different areas of a meander you can plot isovels - lines of equal velocity.

Alternatively you can display these on a map to show isovels along a short stretch of a meandering river.

Cross profiles

A good visual way to get at the cross-profiles that you have drawn onto one page is to arrange them around a sketch map of the meander that you have studied.

Statistical tests

If you have collected data from at least 10 meanders (e.g. Sinuosity Index, meander wavelength, meander amplitude), you can test the significance of its relationship to distance from the river's source by calculating the Spearman's Rank Correlation Coefficient. Need more information about this test?

photography

Sampling techniques

Coasts

Microclimate

Rivers

This section introduces a range of techniques that you can use for fieldwork in river environments.

Rivers

Ecosystems

Rural investigations

Human impact studies

Tourism and recreation

Urban and settlement

Fieldwork technology »

Transport

Investigating opinions

As with coasts, these techniques can be used in the traditional way to study and analyse geographical processes and landforms. Alternatively, why not update your fieldwork slightly to investigate one of the topical and relevant issues in the list below, using the same set of techniques. 

River investigations - why not try...?

Projections - Using the data you collect, draw sketches of how the river would have looked in the past. Make predictions about what it might look like in the future. Annotate

Flood management - Tie your data into the issue of flooding, suggest possible management strategies based on your findings about the channel, discharge and velocity

Hypothetical questions - Investigate a ‘what if...' hypothetical question, for example ‘what if the river was straightened at point A? How would it affect discharge? Velocity? Bed-load? sediment?'

Analysing the methods, for example, the many ways of investigating pebble roundness. Which is the easiest to use? Which is the least biased? Try the methods and compare

Cross-sections

Aims

To investigate the shape and morphology of a river To compare straight and meandering sections of the same river To investigate discharge and velocity and the factors which influence it,

both across the channel and along its length To investigate changes in channel morphology along the length of the river To compare rivers in different locations

Equipment

Tape measure (long and waterproof) Meter ruler (ranging poles can also be used) Waders Data collection sheets

Methodology

Channel width

Stretch a tape measure taut across the river at 90o to the channel. The start and finish points of the tape will depend on whether you are investigating the river in its existing state (see 1) or wish to take into account the conditions when in flood (see 2)

1. To measure current water level, keep the tape about 20cm above the water

level and measure to point where the dry bank meets the water (observe from straight above)

2. To measure the bank-full width - measure to the full height of the bank and width of the river (where the gradient of the bank and vegetation suggest maximum capacity, above which the river would burst its banks and flood)

Figure one: Measuring channel width. (Photo copyright Anne Vaughan).

River depth

Use a meter ruler or ranging pole and take measurements at regular 30cm to 50cm intervals (depending on the channel size).

Figure two: Measuring river depth. Photo copyright Amy Hatchwell).

Wetted perimeter

The wetted perimeter of a river refers to that part of the channel that is in contact with water. It represents the friction that slows down the river velocity, so the longer the wetted perimeter, the more friction between channel and water. Wetted perimeter can be measured using a heavy chain, rope or measure tape, which should be stretched across the river bed from one bank to the other. This can be

hard to do, especially in larger channels or where the bed is very rough. Fast flowing water conditions can also be problematic. Wetted perimeter is often better calculated from the graphed results of the profile.

Figure three: Measuring the wetted perimeter. Photo copyright Anne Vaughan)

Considerations and possible limitations

A soft river bed can affect values. Ensure that the ruler just touches the bed A strong current or bow wave created by the ruler can give inaccurate

depth readings. Ensure narrow edge faces upstream to reduce resistance Large boulders or debris, take care to record any anomalies in depth

caused by irregularities in the river bed

Using the data within an investigation

Raw data can be used to draw a cross-section on graph paper (keeping the same scale for both axes if possible)

Figure four: A river cross section shown in graphical format.

Calculate the wetted perimeter (see method above) and cross-sectional area (width x depth, given in m2) of the river

Channel efficiency can be calculated. This is the cross-sectional area divided by the wetted perimeter and gives an index value (no units) which gives an indication of the river's ability to maintain energy whilst transporting material. The higher the value, the more efficient the river. Differences downstream can be analysed

Comparisons can be drawn between data collected a) from different sections of the same river, or b) from different rivers

The gradient can be examined by conducting a long-profile down the length of the river. Ranging poles are positioned at equal distances upstream and downstream of the cross-section sites - these can be quite far apart while still being easily visible. Slope angle is found using a clinometer (see coasts section on beach profiles). Results can be related to information on velocity, bedload/sediment and efficiency to look for relationships

Velocity

Aims

To investigate changes in the discharge of the river along its length To compare the discharge of rivers in different locations and environments To investigating patterns across a river channel or length To investigate how the human management of rivers can affect discharge

and velocity

Float method

Equipment and methodology

Floating object, for example an orange Tape measure and stop watch

Record the time taken for the object to travel over a set distance.

Considerations and possible limitations

The float used must be visible, durable and not be affected by wind Be aware of possible user error meaning that the start or finish of the object

placement is not exact. Throwing or pushing the object can affect results. Placing the object up-stream and having start and finish lines (tape measures) can help to minimise these errors

This method only records surface velocity Repeated measurement and taking averages can reduce the margin of

error

Flow vanes or meters

Equipment and methodology

A flow meter or vane Tape measure across river (used for cross-section) Waders Record sheets

Different models work differently and should come with instructions. All flow meters record the number of revolutions as water passes over the mechanism. Velocity should be recorded over a period of time, one minute for example, and repeated to obtain averages. Take recordings at different widths (horizontal) and depths

(vertical) across the channel.

Figure five: Using a flow meter to record the velocity of the river. (Image copyright Rebecca Stokes).

The geography   fieldwork pages  of the Barcelona Field Studies Centre provide further information on this topic.

Considerations and possible limitations

The operator can affect readings. Stand beside or down-stream of the flowmeter

Very fast or slow water can affect the accuracy of readings, take multiple readings and average results

Using the data within an investigation

Can be used in isolation or combined with other data, for example: River discharge can be calculated using cross-sectional area and average

velocity as follows:o Discharge = cross-sectional area (m2) x velocity (m/s) = m3 / s.

Visit The Geography Site for more details on using your data One river can be investigated or two or more compared Human influence on discharge can be investigated by also examining the

catchment characteristics

Sediment analysis

Aims

To investigate changes in the sediment and bed-load of a river along its length

To compare sediment changes across the width of the river, particularly across meanders

To compare the sediment load of rivers in different locations or environments

To investigate the possible origins of sediment and bed-load in a river channel

To relate river bed-load and sediment to past and current physical and environmental conditions

Equipment

Ruler or callipers Roundness or angularity charts/indexes (see below) Record sheet

Figure six: Using callipers to measure river bedload.

Methodology

A popular technique is to use the ‘stations' for the depth readings across the channel as sample points for sediment analysis.

1. Reach down with the index finger extended and select the first pebble it touches.

2. Measure the length of the longest axis on this pebble3. Repeat this process, perhaps 10 times per location, ensuring that the

distance from the bank is recorded

4. Analyse your findings using a roundness index or chart

There are various charts and indexes for analysing the roundness or angularity of pebbles, including the different sediment shape and roundness indexes developed by Powers, Cailleux and Zingg. Information about these various methods can be found on the Science Education Resource Centre website.

Considerations and possible limitations

Consideration needs to be given to the size of the sample and the method used to select pebbles in order to avoid bias

The use of visual charts such as Power's can be subjective - one person's opinions on the roundness or angularity of a pebble may differ from another's

Using the data within an investigation

If using Cailleux, the roundness index for each location can be calculated by using the formula:

R = 2r x 1000 / L

where R = Cailleux roundnessr = average radius of curvature (obtained from chart)L = average length of pebbles (in sample)

Things to investigate

Differences in the bed-load of a river as you move from source to mouth Differences across the channel itself could be analysed, for

example across meanders The different bed-rock or parent material of catchments will affect the

characteristics of river load, and could be compared Human interference or management of the river may also affect the river's

ability to transport and erode material

The size and sorting of material on river beaches and slip-off slopes can also be investigated, both down-beach and down-stream.

Suspended load can be investigated by using two litre plastic drinks bottles. Insert a tube into the opening of each bottle and anchor at each site facing upstream (take care to stand downstream when setting up). Leave the bottle for a set period of time and collect. Leave it to stand for a few minutes and comment on sediment characteristics, for example colour, thickness of collected layer, water clarity, settling rate. The sediment can also be filtered and dried out to more accurately assess weights collected at different sites or under different conditions.