Name: ___________________________

Lab 11: Alluvial sediment and soil samples analysis

MATERIALS NEEDED: Scale, squirt bottles, sieves, pestle and mortar, and Munsell soil color chart provided.

You will have to analyze five samples: two of soil and three of alluvial sediment.

QUESTION 1: TEXTURE (for all 5 samples)

Before you start the sieving process, you have to assess the texture of each one of your samples. Using the key provided in the first portion of this lab, estimate the textural class of each sample. Use the squirt bottles provided to moisten the samples. Record your results in the tables on next page.

QUESTION 2: COLOR (for soil samples only)

Using the Munsell soil color chart, carefully select the color most closely matching that of both the dry soil samples. Record your answers in the soil table on next page.

NOTE: YR does not mean years. It is just a letter code for a set of colors.

QUESTION 3: SIZE FRACTION FOR EACH SAMPLE (for all 5 samples)

Procedure to follow for each sample:

1. Remove all visible roots, twigs, leaves, and other organic material from the sample; discard these.

2. Gently break down any soil aggregates in the sample using the mortar and pestle (this will be necessary only for the soil samples).

3. Weigh the sample on the balance and record this weight. To do that, you have to weigh the empty soil tin and subtract this weight from the weight of the total sample.

4. Using the sieves, separate the different size fractions of the sample. This is done by shaking the sieves gently and catching the material that passes through in the basin that fits on the bottom of the sieves. Do not force material through the mesh with your hand, as this will distort your results and may distort the sieve as well. If you see that some soil aggregates remain on the sieve, pour the sieve contents back into the mortar and break these down further, then pass them through the sieves.

5. First, weigh the fraction larger than sand size (the material remaining on the top sieve). Do not forget to always remove the weight of the empty soil tin from your total weight for each size fraction. Then calculate the percentage of the sample coarser than sand (this specific size fraction divided by the total weight of the sample multiply by 100). Repeat the same procedure for each size fraction.

6. Using the triangular texture diagram provided in the notes for this lab, check the accuracy of your hand texturing for the soil samples. To do that, combine the weight of the gravel+coarse+medium fractions as the sand fraction. The fine fraction will be the silt. And the very fine will be the clay.

 

SOIL SAMPLES

 

Sample Horizon A

Sample Horizon B

Texture

Soil weight (g)

% of weight

Soil weight (g)

% of weight

Total weight  
Gravel   (top sieve)  
Coarse fraction  
Medium fraction  
Fine fraction  
Very fine fraction (bottom pan)    
Hand texturing    
Texture diagram    
Color    

NOTE: Make sure that your percentages add up to ~100% and that your size fraction weights add to your total weight.

 

ALLUVIAL SEDIMENT

 

LEFT BANK

CENTER

RIGHT BANK

Texture

Sediment weight (g)

% of weight

Sediment weight (g)

% of weight

Sediment weight (g)

% of weight

Total weight  
Gravel   (top sieve)  
Coarse fraction  
Medium fraction  
Fine fraction  
Very fine fraction (bottom pan)  
Hand texturing      

3. Pool your data for the alluvial sediments with the other teams. Make sure that you report the position of each team along the section we have been studying.

Position of the team

Upstream Group

Middle Group

Downstream Group

Position of the sample

Left

Center

Right

Left

Center

Right

Left

Center

Right

Size fraction

%

%

%

%

%

%

%

%

%

Gravel fraction                  
Coarse fraction
Medium fraction
Fine fraction
Very fine fraction

 

QUESTION 4: DISCUSSION

1. Describe how the texture of the soil samples varies according to the horizon from which they have been collected (A or B)?

 

 

 

 

2. Describe how the color of the soil samples varies according to the horizon from which they have been collected (A or B)?

 

 

 

3. Based on the explanations given in the text at the beginning of this lab, how can you explain this difference in texture and color between horizons A and B?

 

 

 

4. How does texture vary between the soil and alluvial sediment samples?

 

 

 

5. Draw a sketch on which you will mark the position of all alluvial samples along the section of Williams Creek where we have collected them. Indicate the size fraction that dominates for each sample (left bank, middle, and right bank).

6. How does the percentage of each size fraction for all alluvial samples vary according to their position:

(you have to use the data in the tables and the sketch to answer this question)

a) From upstream to downstream?

HINT: Is the size getting finer or coarser?

 

 

 

 

b) From the left bank, to the center, and to the right bank?

HINT: Do you observe a specific spatial pattern (coarser on one bank, finer on the other one, etc.)?

 

 

 

 

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